1. Positive inotropic responses to alpha- and beta-adrenoceptor agonists of isolated papillary muscles from the tree shrew (Tupaia), were compared with those from guinea-pigs. 2. In Tupaia, the concentration-response curve for phenylephrine, unlike that for isoprenaline, was not affected by pindolol in a concentration (10(-8) M) sufficient to block beta-adrenoceptor-mediated responses, but it was significantly shifted to the right by phentolamine (10(-6) M). In guinea-pig papillary muscles, however, the concentration-response curve for phenylephrine, like that for isoprenaline, was shifted to the right by pindolol (10(-8) M) but was unaltered by phentolamine (10(-6) M). Furthermore, when the mean concentrations of agonists inducing maximal positive inotropic responses were compared (relative to that of isoprenaline = 1.0), phenylephrine was found to be only slightly less potent (0.84 +/- 0.04; n = 5) in Tupaia and much less potent (0.33 +/- 0.06; n = 5) in the guinea-pig. 3. Although in Tupaia papillary muscles the increase in developed tension induced by a combination of phenylephrine and isoprenaline did not significantly differ from that by phenylephrine alone, it was approximately 3 times larger than that produced by phenylephrine alone in guinea-pigs. 4. These results indicate that in papillary muscles from Tupaia, unlike the guinea-pig, the positive inotropic effects of phenylephrine can be mediated by alpha-adrenoceptors.