Intake, digestion and growth were examined in young Merino×Border Leicester wether sheep held for 44 days in either cool (13–15°C, thermal humidity index 56–58) or hot (32–40°C, 50–70% relative humidity, thermal humidity index minimum 83–84, maximum 83–88) environments. The sheep were offered diets of medium quality hay ad libitum alone (Con) or supplemented with either 22 g air-dry/kg metabolic liveweight (W0·75) of barley grain fortified with urea and sulphur (Bar/N) or 10 g air-dry/kg W0·75 of fishmeal (FM). Intake of the Con diet by the sheep in the cool environment was high at 79 g DM/kg W0·75 per day. Sheep in the hot environment had higher rectal temperatures and higher respiration rates (40·1°C v. 39·2°C, 196 v. 56 respirations/min respectively, P<0·01). The hot environment reduced (P<0·05) total dry matter (DM) intake, estimated metabolizable energy (ME) intake, liveweight (LW) gain and nitrogen (N) balance. The provision of supplements did not change total DM intake, but increased (P<0·05) organic matter digestibility, estimated ME intake, LW gain and N balance. Wool growth was increased much more by the FM than by the Bar/N supplement, indicating that the supply of absorbed amino acids was increased substantially by the FM supplement. Neither voluntary intake nor productivity were influenced by any interactions between the thermal environments and the balance of nutrients provided by the diets. In conclusion, in these young sheep consuming a high intake of a medium quality roughage diet, moderate heat stress reduced intake and growth but did not affect the relative responses of the sheep to supplements providing principally fermentable ME or a similar amount of fermentable ME and additional metabolizable protein.