Distribution in kidney and transmigration to fetus or suckling in rats were studied in male, pregnant or lactating rats after intramuscular administration of 14C-netilmicin (20 mg/kg). 1. After administration to male rats, the radioactivity in the kidney declined slowly with a half-life of approximately 6 days. 2. The radioactivity in the kidney was distributed in the renal cortex. The distribution pattern was further investigated by means of microautoradiography. The radioactivity was specifically observed in lysosomal granules of the proximal tubules at 6 hours after administration and reached maximum at 24 hours after administration, then declined gradually. On the other hand, the radioactivity in the distal tubules was lower than that of the proximal tubules. The highest radioactivity in the distal tubules was detected later than 24 hours after administration. 3. In pregnant rats (20th day of gestation), the distribution of radioactivity in the tissues were almost the same as those in the male rat. The small amount of radioactivity was detected in the fetal kidney and bone. 4. In mother rats (14 days after parturition), the radioactivity in the milk was 3 approximately 4 mcg equivalent of netilmicin/ml during 24 hours after administration. The small amount of radioactivity (0.13% of dose) was observed in the gastrointestinal contents of a suckling with 6 hours after administration.