I. 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 zoo 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-weight 0"6 per d at 28O.6. The critical temperature of early weaned piglets raised in intensive modem housing and fed at about go % of the ad lib. intake is close to 28" during the first 12 d after weaning.Considerable attention has been given during the last decade to the influence of environmental temperature on energy metabolism in the newborn pig and the growing pig (Holmes & Close, 1977). In contrast, there is little available information on the energy metabolism of piglets immediately after weaning in spite of the likely effects of thermal environment on performance (Le Dividich & Aumaitre, 1978). The thermal insulation of 3-4 weeks old piglets is poorly developed and modern intensive housing systems, particularly those of early weaning in totally wired cages, does not afford protection against adverse climatic conditions. Recently, Close & Stanier (1979) studied the effect of environmental temperature on energy and nitrogen metabolism in early weaned piglets. However, the authors did not take into account the immediate period following weaning. From a nutritional point of view, this period is associated with a temporary under-feeding (Leibbrandt et al. 1975a) so that the heat increment of feeding available for maintaining homoeothermy in the cold is limited. The question arises therefore as to the extent to which energy and protein intake contribute to the energy requirements for thermoregulation and growth in piglets exposed to cold environmental conditions immediately after early weaning.The purpose of the present experiment was to investigate the effect of environmental temperature on energy and N metabolism of piglets immediately after early weaning, i.e. without any habituation to the experimental treatments. To make the conditions more closely comparable with those of the animals' normal mode of rearing, piglets were...