Pharmacokinetics of 14C-labeled montirelin hydrate (CAS 90243-66-6, NS-3) in rats was studied after single or repeated intravenous administration. 1. Radioactivity concentrations in tissues after single administration to male and female rats were highest in the kidney followed (in this order) by plasma, liver, blood, pancreas, uterus (female rats), lung and skin and low in various brain sites 5 min after administration. The concentrations in most tissues were practically parallel to those in plasma over the 24-h period after administration. After decreasing rapidly the concentrations rose slightly for 10 h and then decreased gradually. 2. Five min after single administration to male rats, the concentration of the main metabolite CNK-6004 (deamidated product) was lower in the plasma, but higher in the liver and kidney than the NS-3 concentration. From 0.5 to 2 h after administration, the concentration of CNK-6004 was higher than that of NS-3 in the plasma, liver and kidney, accounting for 33-64% of the radioactivity concentration. 3. After administration to rats on the 18th day of pregnancy, the radioactivity concentrations in the fetal whole body and fetal tissues peaked later than those in the maternal plasma, tissues and placenta. The maximum concentration in the fetal tissues was 2% or less of that in the maternal plasma. 4. After administration to lactating rats, the radioactivity concentration in milk reached the maximum 10 h after administration, and decreased gradually in parallel with the concentration in the plasma 24 to 168 h after administration. 5. During repeated once daily administration for 10 days, the radioactivity concentration in the plasma 24 h after each administration reached practically steady state level after the 7th administration and decreased with a half-life of 38.1 h after the last administration. 6. The radioactivity concentrations in most tissues after the last administration were not significantly different from those after a single administration. Decreased elimination of the radioactivity was observed in the white fat and skin, in which radioactivity levels were higher than those in the other tissues a long time after the last dose. 7. Excretion of the radioactivity in the urine and feces during repeated administration was constant after the 2nd administration. The excretion by 168 h after the last administration was 66.9 and 14.3% of the cumulative dose in the urine and feces, respectively (total: 81.2%). 8. The composition of metabolites in the plasma, liver, kidney and urine after the last administration did not differ markedly from that after a single administration. Once daily repeated administration for 7 days had no effect on the liver drug metabolizing enzyme activities.