To discover whether humanlike active skin vasodilation occurs in a sweating, subhuman primate, four unanesthetized male baboons (Papio anubis) were heated (blood temp = 39-39.5 degrees C) before and after arterial administration of the alpha-adrenergic blocking agent phenoxybenzamine HCl. Hcat stress alone increased common iliac vascular conductance (CIVC) from 1.2 to 3.7 ml-min-1-mmHg-1 (averages). alpha-Blockade alone increased CIVC from 1.2 to 3.3 ml-min-1-mmHg-1. Heat stress subsequent to alpha-blockade further increased CIVC by only 0.4 ml-min-1-mmHg-1. Thus, most of the rise in CIVC caused by heat stress could be mimicked by alpha-blockade. If, as in man, the dominant means of raising skin blood flow were nonadrenergic active vasodilation, the response of CIVC to heating would far exceed that due to alpha-blockade. We conclude that, in baboons, humanlike active vasodilation of skin plays no significant role in CIVC response to heat stress. Thus, the baboon is not an appropriate model for investigation of control of human skin circulation during hyperthermia.