The metabolic responsiveness of spontaneously hypertensive (SH) and Wistar-Kyoto (WKY) normotensive rats to administration of the beta-adrenergic agonist, isoproterenol, was assessed by changes in tail-skin temperature, heart rate, and plasma glucose concentration after administration of graded doses of isoproterenol. The increases in mean tail-skin temperature accompanying administration of isoproterenol at doses of 5, 10, and 40 micrograms/kg sc were significantly (P less than 0.05) greater for SH than for WKY rats. Resting colonic temperatures of the SH rats were also significantly (P less than 0.01) higher than those of the WKY normotensive rats. Mean maximal heart rates of unanesthetized SH rats after administration of isoproterenol at doses of 5, 10, and 20 micrograms/kg sc increased to levels significantly (P less than 0.05-0.01) higher than those of WKY normotensive rats. Resting heart rates were similar in both groups. Increases in plasma glucose concentration after administration of isoproterenol at doses of 5, 10, and 20 micrograms/kg sc were significantly (P less than 0.05) greater in the SH rats compared with the WKY rats. These results suggest that the SH rat has a significantly greater metabolic responsiveness to beta-adrenergic stimulation, as assessed by three separate tests of beta-adrenergic function, than does the WKY rat.