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The absorption of terramycin from the gastrointestinal tract is rather similar to that of aureomycin. Peak levels are low, tend to be achieved in 2 to 6 hours after ingestion, and are prolonged for several hours. Single oral doses of 11 mg/kg. body weight give peak serum levels ranging from 0.20 to 1.95 µg./cc., with a mean of 1.0 µg./cc. Doubling or tripling this single dose did not produce marked increases in serum levels. Dosage of 11 mg./kg. orally every six hours (44 mg./kg./24 hours) resulted in peak serum levels on the third day which were essentially identical with those after the initial dose. However, when a dosage of 33 mg./kg. was given orally every six hours (132 mg./kg./24 hours), marked cumulation in the serum was noted. The mean serum level on the first day on this schedule was 2.7 µg./cc., and on the third day, 8.0 µg./cc. Diffusion into the cerebrospinal fluid in three cases was poor. With spinal fluid and serum levels simultaneously obtained, the spinal fluid levels were ¼, ⅙ and ⅛, respectively of the serum levels. Intravenously administered terramycin in dosage of 6.6 mg./kg. gave peak serum levels at one-half hour, ranging from 10.0 to 12.8 µg./cc. Six hours after injection, serum levels ranged from 1.8 to 4.4 µg./cc., and at 12 hours, from 0.78 to 2.6 µg./cc. Terramycin given rectally in perforated capsules was well tolerated. However, in four patients given 66 mg./kg. by this route, peak serum levels ranged from 0 to 1.0 µg./cc. Recommended terramycin dosage: (a) Oral: 11 mg./kg./dose every six hours (44 mg./kg./24 hours). Adequate antibacterial serum levels for most susceptible organisms are produced with this dosage. Increasing the dosage to 33 mg./kg./dose every six hours (132 mg./kg./24 hours) may be done in cases when high serum levels seem indicated. (b) Intravenous: 6.6 to 11 mg./kg./dose every 6 to 8 hours.