TRH (10 and 1000 micrograms/kg body weight (BW] was injected ip into pregnant rats daily from day 0 to 20 of pregnancy, and the pituitary-thyroid axis of their pups (Mat-TRH rats) was examined on days 0, 4, 10, 21 and 90 after birth. The pituitary TSH content of male Mat-TRH rats was significantly lower on day 4, and higher on day 10 than that of control rats. The serum TSH was significantly higher on day 10 (except female 10 micrograms/kg group). An exaggerated TSH response to exogenous TRH (10 micrograms/kg BW; ip) was observed on day 10 (males, 1000 micrograms/kg group). The serum T4 level of female Mat-TRH rats was low on day 4 (1000 micrograms/kg group), and higher on day 10. On days 21 and 90, the levels of pituitary TSH, serum TSH and T4 in Mat-TRH rats were similar to those in controls, but the TSH response to TRH was still exaggerated (1000 micrograms/kg group). No significant difference between control and TRH-treated mothers was seen on days 10 and 90 postpartum except for a decreased pituitary TSH content on day 10 in the 1000 micrograms/kg group. It is concluded that repeated administration of TRH to pregnant rats shows an effect on the pituitary-thyroid axis function of their progeny in later life.