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1. The metabolism of sulphanilamide, sulphadimidine (4,6-dimethyl-2-sulphanilamidopyrimidine), sulphamethoxazole (5-methyl-3-sulphanilamidoisoxazole) and sulphadoxine (5,6-dimethoxy-4-sulphanilamidopyrimidine) given by intravenous injection has been examined in cows. 2. The sulphonamides were present mainly as unchanged drugs in blood samples collected 2h after administration. 3. The sulphonamides were excreted in the milk partly as unchanged drugs and partly as conjugated metabolites whereas only small amounts were excreted as the N(4)-acetyl derivatives. 4. The unchanged drug and the N(4)-acetyl derivative were the major constituents in urine samples after administration of sulphanilamide, sulphamethoxazole and sulphadoxine. 5. Besides the unchanged drug, the N(4)-acetyl derivative and the conjugated metabolites, three further metabolites of sulphadimidine were isolated from urine samples and identified. They were 5-hydroxy-4,6-dimethyl-2-sulphanilamidopyrimidine, 4-hydroxymethyl-6-methyl-2-sulphanilamidopyrimidine and sulphaguanidine.

The distribution of a highly bound antibacterial sulfonamide was markedly altered in both the mother rat and its fetus by interfering with the binding of this drug to plasma protein in the mother. This effect was due to binding displacement, since the displacing agent had little or no effect on the distribution of another sulfonamide with very low binding to plasma protein.