Epidermal growth factor (EGF) liver receptor ontogeny and somatic growth were studied in mice from day 7 to day 70 postnatally to assess long-term effects of short-term postnatal thyroxine treatment. The mice were given 0.4 micrograms thyroxine/g body weight/day for the 1st wk of life. EGF receptor binding in liver tissue was studied on days 7, 15, 20, 30, and 70 postnatally. Treated animals had accelerated eyelid opening and tooth eruption, and permanent growth retardation was obvious from the second week of life. Hepatic EGF receptor-binding capacity increased markedly in control mice with increasing age in contrast to a very slow increase in treated mice, making the difference statistically significant (P less than 0.01) from day 30. The affinity of EGF receptor binding initially was similar in the two groups of animals (1.09 x 10(9) M-1 and 1.02 x 10(9) M-1) and increased by day 30 in controls (2.57 x 10(9) M-1), an increase that was not observed in treated animals either at day 30 or 70. These results suggest a sensitive period of imprinting during the first 7 days postnatally, a period when thyroxine can exert a permanent effect on later growth and later hepatic EGF receptor number.