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This thesis consists of two parts. The first is a clinical and pathological description of the different types of nephritis in the dog; the second is a study of experimental interstitial nephritis. Part I A study of the different types of nephritis in the dog. The 178 cases encountered in this survey were classified on a histopathological basis. 74% more interstitial nephritis, 11% glomerulonephritis and allied conditions, 5% embolic or pyaemic nephritis and 10% pyelonephritis. Section 1 Interstitial nephritis. These cases have been further classified into acute and chronic. 20% were associated with other diseases. Acute nephritis was more prevalent among the younger animals while chronic nephritis was present among the older ones. The incidence was three times higher in the male than in the female. Most, if not all breeds appear to be susceptible. Schuffner tests indicated that all acute cases gave agglutination titres of 1 in 10,000 or higher, while chronic cases gave low titres of 1 in 3,000 or lower to Leptospira canicola. 32% of the chronic cases gave negative titres. Hametological examination revealed an increase in the oxythrocyte sedimentation rate with the chronicity of the illness. No evidence of an anaemia was seen in the acute cases, while 45% of the chronic ones showed evidence of a secondary anaemia mainly of the macrocytic type. A leucocytosis accompanied by a neutrophilia was observed in 90% of the acute and 52% of the chronic forms. Biochemical investigations revealed that 100% of the acute and 97% of the chronic cases showed a retention of nitrogen in the blood. Low urine levels were seen in 33% of the acute and 95% of the chronic cases. All cases with low urine urea levels were associated with an azotaemia. Microscopically the cellular reaction of predominantly mononuclear cells of the lymphocytic and the plasma cell series in acute cases suggest a local immune reaction rather than an inflammatory response. The effect of the cellular reaction appeared to be one of tubular compression and destruction. Chronic cases are characterized by fibrosis of the interstitial tissue while the cellular reaction is less marked. This probably represents a later stage of the immune reaction. The disease had progressed to produce an osteodystrophia fibrosa in 10% of the cases of interstitial nephritis. The bony changes were clinically manifested as "rubber jaw". These cases had a severe chronic interstitial nephritis and a marked hyperplasia of the parathyroid glands. Section 2 Glomorulonephritis and allied conditions 11% of the cases showed primarily a glomerular lesion and were comprised of 1.7% nodular glomerulonephritis, 2.8% both primary and secondary amyloidosis, 3.9% glomerulonephritis of the amyloid type and 2.8% glomerular lipidosis. In nodular glomerulonephritis a solid nodulation of the capillary tuft was the characteristic lesion and these cases were associated with a marked proteinuria. Renal amyloidosis was associated with s massive proteinuria. Secondary amyloidosis occurred secondary to a tuberculous infection. There exists a type of lesion morphologically similar to renal amyloidosis which differs in its negative metachromatic reaction to methyl violet. These were termed glomerulonephritis of the amyloid type. A fatty nodule formed of foam cells were present in the capillary tuft in glomerular lipidosis. Unlike the above types this lesion were associated with a mild proteinuria. Section 3 Embolic or pyaemic nephritis 5% cases showed a suppurative nephritis. They were associated with suppurative lessions a haematogenic mode of infection was inferred. Section 4 Pyelonephritis 10% cases showed a pyelonephritis. The mode of infection on gross and microscopic evidence appeared to be haematogenous. All cases were associated with a concurrent infection. Obstruction in the urinary tract both partial and complete plays an important role in the pathogenesis of the disease. Eschrochia coli was isolated from a number of cases. One case of tuberculous pyelonephritis is reported. Typical tuberculous lesions present in the kidney. Unlike interstitial nephritis due to L. canicola the renal lesion in cases of embolic and pyelonephritis is a true inflammatory response characterised by the infiltration of the interstitium by inflammatory cells. Part II Experimental interstitial nephritis 9 pups were inoculated with a suspension of L. canicola. All animals developed agglutinins but showed individual variations. High titres were attained within a fortnight but lasted for about three weeks after which the titres gradually fall. The leptospiraemic phase occurred within the first four days after inoculation. This phase was characterised by an elevation of temperature and also the absence of circulating antibodies. The leucocytosis observed was mild to moderate. The renal lesions produced were mild focal ones, they resembled the naturally occurring ones. No lesions were demonstrable in two instances. Attempts to produce severe lesions within the dosage range employed failed.