The impact of heat stress on the feed intake, milk production, water intake and urine output of undernourished lactating ewes and on the growth rate and water intake of their lambs was measured both in a climate chamber and during exposure to natural high ambient temperature conditions. Similar liveweight increases occurred in both stressed and unstressed lambs. Freed intake was depressed in heat-stressed ewes in the climate chamber but not under natural heat-stress conditions. During the first and second weeks of lactation calculated milk yield (200--500 ml/day) and composition were unaffected by heat stress per se. However, undernutrition due to the poor quality roughage offered apparently depressed milk production of all ewes. Increased water intake (27%) and plasma prolactin concentrations (220%) were recorded in heat-stressed ewes, but daily urine output (27--36 ml/kg body wt) was unaffected. No relationship between prolactin concentrations, milk production and antidiuretic activity was obvious. At the age of 5--6 weeks water intake accounted for 67% (500 ml/day) and 80% (1000 ml/day) of total fluid intake (water plus milk) of unstressed and heat-stressed lambs respectively. These data indicate the importance of making water freely available to lambs subjected to extensive grazing systems of tropical regions.