The effects of the selective alpha 2-adrenoceptor antagonists yohimbine and its stereo-isomer rauwolscine and the selective alpha 1-adrenoceptor antagonist corynanthine (a third yohimbine stereoisomer) on contractions induced in rat aorta by depolarization and prostaglandin F2 alpha (PGF2 alpha) have been compared. In calcium-free solution, depolarization with 100 mM K+ failed to produce a contraction of rat aorta but PGF2 alpha (3 microM) stimulated a contraction equal to about 23% of maximal elicited in normal physiological solution. Yohimbine had no significant effect on depolarization-induced contractions except at concentrations greater than 30 microM. Rauwolscine and corynanthine (1 to 100 microM) depressed depolarization-induced contractions in a concentration-dependent manner, but the characteristics of inhibition were not identical. Contractions induced by PGF2 alpha (3 microM) were depressed in a concentration-dependent manner by rauwolscine (3 to 100 microM) but were unaffected by yohimbine or corynanthine. Depolarization-stimulated 45Ca influx was depressed by rauwolscine and corynanthine to about the same extent as were the contractions; while rauwolscine (100 microM) completely inhibited PGF2 alpha-stimulated 45Ca influx, it also depressed part of the PGF2 alpha-stimulated contraction dependent on intracellular calcium. Rauwolscine (100 microM) partly inhibited PGF2 alpha-stimulated release of 45Ca from aortic smooth muscle in calcium-free solution. It is concluded that the yohimbine structure possesses a calcium entry blocking action as well as a depressant action on contractions not dependent on calcium entry. The predominant effect depends on the structural configuration and the nature of the stimulating agent.