Activities of erythrocyte aldose reductase were compared in 34 normal subjects, 45 diabetic patients, and nine young men following immersion in water at 25, 39, and 42 degrees C. Mean basal enzyme activity was 1.11 (SEM 0.12) U/g Hb and 2.07 (SEM 0.14) U/g Hb in normal controls and diabetic patients, respectively (P < 0.0001). Activities of the enzyme showed a good correlation with hemaglobin A1 (HbA1) concentrations (P < 0.01) but not with fasting plasma glucose concentrations. After immersion at 42 degrees C for 10 min, enzyme activity was increased by 37.6% (P < 0.01); however, the activity decreased by 52.2% (P < 0.005) after immersion for 10 min at 39 degrees C and by 47.0% (P < 0.05) at 25 degrees C. These changes suggest that heat stress might aggravate diabetic complications, and body exposure to hot environmental conditions is not recommended for diabetic patients.