AbstractEffects of the inhibition of maternal phenylalanine (Phe) or tyrosine (Tyr) hydroxylase on the postnatal maturation of catecholamine, thyroxine and amino acid metabolisms were examined in young rats. The intraperitoneal injections of 100 mg/kg p‐chlorophenylalanine three times during the second gestational week together with 0.2% Phe solution as drinking water produced marked reduction in birth weight, body weight gain, brain weight, cerebral Tyr hydroxylase activity, adrenal epinephrine, renal norepinephrine and free amino acids in plasma and brain as well as delayed maturation of thermoregulation in the offspring. In contrast, the administration of 100 mg/kg α‐methyl‐p‐tyrosine with 0.2% Tyr supplementation did not affect general growth, but only decreased cerebral Tyr hydroxylase and serum thyroxine level, while cerebral dopamine was increased. These results indicate that maternal Phe hydroxylase inhibition induces more severe growth failure including catecholamine and amino acid metabolism than Try hydroxylase inhibition in postnatal dev