1. Antecubital venous plasma glucose and insulin concentrations after ingestion of 75 g of glucose were higher in six normal subjects when studied at an ambient temperature of 33 degrees C at an ambient temperature of 23 degrees C; the mean area under the glucose-time curve increased from 833 at 23 degrees C to 990 mmol min-1 at 33 degrees C, that for insulin from 5300 to 7900 m-units min 1-1. 2. Core temperature was elevated by 0.5 degree C at 33 degrees C ambient, although there was no marked stress response as judged by plasma levels of catecholamines, cortisol and growth hormone; at 2 h after glucose ingestion, mean noradrenaline levels were lower at 33 degrees C than at 23 degrees C (1.1 at 33 degrees C vs 1.8 nmol/l at 23 degrees C), adrenaline slightly higher (0.18 at 33 degrees C vs 0.09 nmol/l at 23 degrees C), cortisol and growth hormone unchanged. 3. Forearm blood flow was markedly elevated at 33 degrees C ambient (mean total flow 9.1 at 33 degrees C vs 1.8 ml min-1 100 ml-1 at 23 degrees C), as were antecubital venous partial pressure of oxygen (mean 10.1 at 33 degrees C vs 5.6 kPa at 23 degrees C) and oxygen saturation (mean 92% at 33 degrees C vs 70% at 23 degrees C). There was a positive correlation between oxygen saturation and area under the glucose tolerance curve. 4. In separate experiments, arterialized glucose concentrations were measured after glucose ingestion at 23 degrees C ambient.(ABSTRACT TRUNCATED AT 250 WORDS)