Gas exchanges were measured in 2, 5 and 7-day old fa/fa and Fa/fa rats for two hours (10.30-12.30 h) at five different temperatures: 35, 33, 30, 28 and 26 degrees C. In 2-day old pups, no significant difference between genotypes was found at either temperatures. At the age of 5 and 7 days, gas exchanges were significantly lower in fa/fa than in Fa/fa pups, but only at temperatures below 33 degrees C. These results differ from those of a previous study where gas exchanges were measured in the afternoon (16.30-18.30 h) (Planche and Joliff, 1985). Thus, our findings show that very early in life there are diurnal variations in the level of gas exchanges according to genotypes. Consequently, estimation of the deficit of energy expenditure in fa/fa rats observed in the present study during the first week of life should be modulated and emphasis laid on the effect of the hour of measurement, even from the early days of life, on the result of metabolic studies.