1. Six experiments each involving two groups of six piglets, were designed to study the influence of environmental temperature on heat production, energy retention and protein and fat gain in early weaned piglets. Immediately after weaning, at a mean age of 25 d, the animals were raised in two open circuit respiratory chambers. Each chamber was equipped with a totally wired cage. The piglets were paired-fed and maintained at environmental temperatures of 20, 24 or 28°. Four replicates were used for each temperature. Metabolizable energy, heat production and nitrogen balance were measured during two consecutive periods (A and B), each of 6 d duration.
2. Heat production was higher at 20° than at 24 and 28° during periods A and B. Energy retention was negative during period A, it was positive during period B and increased with temperature.
3. Protein deposition was always positive and independent of environmental temperature. The net efficiency of protein utilization was 0.77.
4. Body fat was mobilized during period A at a higher rate at 20° than 28°. During period B, fat gain increased with increase in temperature.
5. The calculated ME requirement for maintenance amounted to 411 kJ/kg body-weight0.75 per d at 28°.
6. The critical temperature of early weaned piglets raised in intensive modern housing and fed at about 90% of the ad lib. intake is close to 28° during the first 12 d after weaning.