Studies of substrate flux, isotope activity and metabolic balance frequently require arterial sampling. We evaluated: (1) whether substrate concentrations obtained from heated dorsal hand veins (HDHV) were comparable to samples obtained from the radial artery, (2) whether heat sufficient to arterialize HDHV altered contralateral forearm blood flow thus affecting flux calculations, (3) whether a +14 heating pad equaled a cumbersome +700 heating chamber, and (4) whether HDHV showed a dose-response curve to varying heat loads. In 12 normals, dorsal hand temperature was raised from 31.8 +/- 0.6 degrees C to 39.8 +/- 0.8 degrees C (chamber) and 39.3 +/- 0.3 degrees C (pad). Basal contralateral forearm blood flow (3.37 +/- 0.7 ml/100 ml tissue/min) was not significantly altered in the chamber (3.39 +/- 0.5 ml) or the pad (3.44 +/- 0.5 ml). Skin temperature of the unheated hand, an index of superficial blood flow (31.5 +/- 0.7 degrees C) did not change significantly in the chamber (31.6 +/- 0.7 degrees C) or the pad (31.2 +/- 0.7 degrees C). Forearm blood flow did not change with heating in eight postoperative patients. Comparative arterial and HDHV blood gases and 10 metabolic substrates from simultaneously drawn samples at various temperatures showed HDHV PO2 approached but did not equal arterial PO2 at temperatures greater than 39 degrees C. Glucose, amino acid, and substrate concentrations were comparable at 39 degrees C and did not change with increasing temperature. HDHV can reliably determine arterial substrate concentrations using an inexpensive heating pad. In cool environments (20-22 degrees C), contralateral forearm blood flow is not significantly altered. There is no benefit to heating the hand above 39 degrees C.