Blood glucose, plasma concentrations of gastric inhibitory polypeptide, insulin, glucagon, cortisol, and thyroid hormones were measured in nonobese and obese human subjects at 30 and 22 degrees C ambient temperature (Ta). Oxygen consumption (VO2), carbon dioxide output (VCO2), and temperatures in the external auditory meatus (Tc) and on the skin surface (Tsk) were also measured. After 1 h, near naked at the chosen Ta, an oral dose of sucrose (approximately 1.5 g/kg) was given and the subjects were then monitored for a further 60 or 90 min. Following sucrose ingestion, both in the nonobese and obese, there were significant (p less than 0.001) increases in the following: glucose, gastric inhibitory polypeptide, insulin, VO2, and respiratory quotient. The effect of Ta on these responses in the nonobese was that gastric inhibitory polypeptide rose more at Ta 30 than at Ta 22 (p less than 0.05) and VO2 rose more at Ta 22 than at Ta 30 (p less than 0.05). In the obese, glucose rose more at Ta 30 than at Ta 22 (p less than 0.02), VO2 rise was less than in the nonobese at Ta 22 (p less than 0.05), and the respiratory quotient was lower than in the nonobese at both Ta 30 and 22 (p less than 0.001). Gastric inhibitory polypeptide changes with respect to Ta in the obese were inconsistent. It is concluded that responses to oral sucrose are modified by environmental temperature.