1. The present studies were undertaken to characterize the thermogenic response to the alpha 1-adrenergic agonist phenylephrine, to compare this response with the beta-adrenergic response and to assess the role of brown adipose tissue (BAT). 2. Phenylephrine and the beta 3-adrenergic agonist CGP 12177A each caused similar increases in O2 consumption. No synergism was observed when combining the two drugs. Phenylephrine stimulated O2 consumption via an alpha-adrenergic mechanism, as indicated by effective blockade with phenoxybenzamine, but not propranolol. 3. Further evidence for the alpha-adrenergic mechanism of phenylephrine action was seen in studies with BAT membranes. Phenylephrine did not stimulate adenylyl cyclase and did not potentiate beta-adrenergic stimulation of adenylyl cyclase. 4. Skeletal muscle was not a major site of phenylephrine-stimulated O2 consumption, as the response was not inhibited by a concentration of dantrolene which inhibited cold-induced muscle shivering. 5. Phenylephrine caused an increase in the density of available 3H-GDP binding sites in BAT mitochondria, indicating an activation of BAT thermogenesis in vivo. This increase was equal in magnitude to what we have reported previously for CGP-12177A. No changes were observed in the affinity for 3H-GDP. 6. We concluded that phenylephrine stimulates O2 consumption by an alpha-adrenergic mechanism that involves activation of BAT thermogenesis. It remains to be determined whether the activation of BAT occurs by a direct or indirect mechanism.