Abstract Various vasoactive peptides were compared for their histamine releasing effects on rat mast cells. Neurotensin, substance P (SP), and kallidin were the most active natural peptides, followed by bradykinin; neurokinin A and B. bombesin, angiotensin and tuftsin were practically inactive. Several kinins and tachykinin-related peptides were tested in an attempt to characterize the receptors mediating histamine liberation. The order of potency of the kinins was the following: kallidin > [tyr(Me) 8]bradykinin = Bradikinin > [desArg 10]kallidin > desArg 9-bradykinin, the same as that found in smooth muscle possessing receptors of the B2 type. Tachykinin-related peptides were potent stimulants and followed the order: [D-Tryp 7,9,10]SP-(1−11) > [D-Pro 2, D-Tryp 7,9,10]SP-(1−11) > SP-(1−9) > [D-Pro 4,D-Tryp 7,9, Leu 11]SP-(4−11) > SP-(1−7) > SP-(4−11) > neurokinin A = neurokonin B, indicating that: (a) undecapeptide antagonists of SP behave as superagonists: (b) both N- and C-terminal portions of SP-(1–11) are essential for activity; and (c) receptors for the tachykinins mediating histamine release appear to be of the SP-P type.