1. Two experiments were conducted to investigate the impact of high temperature and dietary tyrosine (Tyr) content on performance and activity of hepatic tyrosine aminotransferase (EC 18.104.22.168.), an enzyme that catalyses the first step in the metabolic degradation of Tyr in broiler chickens. 2. Two-week-old birds were allocated to one of three temperature treatments: 24°C (control), 36°C (heat stress, HS) and 24°C pair-fed (24PF) for 2 weeks and fed on diets containing 100% (Experiment 1) and 50, 100 and 200% (Experiment 2) of the NRC requirement for Tyr. 3. In Experiment 1, exposure of chickens to 36°C for 2 weeks caused significant increase in hepatic tyrosine aminotransferase activity but no significant change in activity of hepatic phenylalanine 4-hydroxylase (EC 22.214.171.124) (an enzyme that catalyses conversion of phenylalanine to Tyr) compared with the 24PF birds. No significant changes attributable to heat stress were detected in hepatic glutamic-oxaloacetic transaminase (EC 126.96.36.199) activity. 4. In Experiment 2, heat stress caused reductions in weight gain and feed intake in chickens on all diets, compared with their control counterparts. Hepatic tyrosine aminotransferase activity was increased by heat stress compared with their 24PF counterparts in chickens fed on the 100 and 200% Tyr diets, while in chickens fed the 50% Tyr diet, it was reduced by heat stress. 5. From these results, it is suggested that hepatic tyrosine aminotransferase activity is affected by heat stress and dietary Tyr content and the increased tyrosine aminotransferase activity with, in part, relatively low phenylalanine hydroxylase activity in hepatic tissues may be involved in the Tyr metabolism characteristic of heat-stressed chickens.