The myotropic effects of bradykinin (BK) and other kinins in two isolated veins, the rabbit jugular and the guinea pig anterior mesenteric, have been studied. The effects of degradation on the biological activities of these compounds and the receptor types mediating their myotropic effects have been determined. It has been found that contractions elicited by kinins in these veins result from direct actions on specific receptors. Both veins contain active kininase II (but not active carboxypeptidases A or B) which interferes with the measurement of the myotropic effects and can be blocked by 1-(D-3-mercapto-2-methyl-3-oxopropyl)-L-proline (SQ 14225). The slopes but not the maxima of the concentration-response curves of BK and other kinins measured in the presence of SQ 14225 are different from those recorded in its absence whereas no significant changes are observed in concentration-response curves obtained with the analogue [D-Phe8]-BK, which resists degradation by kininase II. BK is more potent than its fragment sequence des-Arg9-BK and the effects of kinins are antagonized by dihydrochlorprothixene and beta-phenylalanine hexyl ester. These findings suggest that the rabbit jugular and the guinea pig anterior mesenteric veins contain receptors of the B2 type. The findings presented in this paper indicate that the rabbit jugular vein and the guinea pig anterior mesenteric vein are sensitive preparations for kinins and they can be useful for structure-activity studies of these peptides. A correct evaluation of the relative affinities of various kinins is, however, only possible by eliminating the interference of kininase II.