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°C than at an ambient temperature of 23°C; the mean area under the glucose-time curve increased from 833 at 23°C to 990 mmol min 1 −1 at 33°C, that for insulin from 5300 to 7900 m-units min 1 −1 . 2. Core temperature was elevated by 0.5°C at 33°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°C than at 23°C (1.1 at 33°C vs 1.8 nmol/l at 23°C), adrenaline slightly higher (0.18 at 33°C vs 0.09 nmol/l at 23°C), cortisol and growth hormone unchanged. 3. Forearm blood flow was markedly elevated at 33°C ambient (mean total flow 9.1 at 33°C vs 1.8 ml min −1 100 ml −1 at 23°C), as were antecubital venous partial pressure of oxygen (mean 10.1 at 33°C vs 5.6 kPa at 23°C) and oxygen saturation (mean 92% at 33°C vs 70% at 23°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°C ambient. Antecubital venous glucose concentrations at 33°C ambient were intermediate between venous and arterialized concentrations at 23°C ambient. 5. It is concluded that the apparent impairment of glucose tolerance at elevated ambient temperature is caused at least in part by increased arterialization of the antecubital venous blood. This potential variable must be considered in research or diagnostic procedures based on the use of venous blood.