Apparent affinities of kinin agonists and antagonists were determined in terms of pD2 and pA2 respectively, on three isolated smooth muscles: rabbit jugular vein (Rb.J.V.), rabbit aorta (Rb.A.) and guinea pig ileum (G.P.I.). Both kinin agonists and antagonists were evaluated for their ability to induce the release of histamine from rat mastocytes. Our results indicate that the kininase I metabolites (desArg9-BK and desArg10-KD) were inactive on Rb.J.V. and G.P.I. (B2 preparations) and were full agonists on Rb.A. (B1) while [Tyr(Me)8]-BK and [Hyp3,Tyr(Me)8]-BK were inactive on Rb.A. and maintain a high affinity on Rb.J.V. and G.P.I. In addition, [Hyp3]-BK was a potent agonist on Rb.J.V. (pD2 = 8.88) and was of a moderate affinity on G.P.I. (pD2 = 7.27). On the other hand, the affinity of [Aib7]-BK was identical to that of BK on G.P.I. (pD2 = 7.90) but drastically reduced in Rb.J.V. (pD2 = 6.28). Conctractile effects of kinins in the Rb.J.V. and G.P.I. were reduced or eliminated by B2 receptor antagonists but at different concentration levels (e.g. DArg[Hyp3,DPhe7,Leu8]-BK showed pA2 values of 8.86 on Rb.J.V., but only 6.77 on G.P.I. DArg[Hyp3,Gly6,Leu8]BK showed high affinity on Rb.J.V. (pA2 = 7.60) but was a full agonist on G.P.I. Conversely, DArg[Tyr3,DPhe7,Leu8,BK] showed high agonistic activity on Rb.J.V. (pD2 = 8.30, alpha E = 1.0) and showed a pA2 value of 6.80 on G.P.I. All compounds (agonists and antagonists) were quite potent on histamine release induced in rat mastocytes. [Arg1(Tos),Hyp3,Thi5,DTic7,Oic8]-BK and DArg[Hyp3,Thi5,DTic7,Oic8]-BK showed almost similar pA2 values on both Rb.J.V. and G.P.I., but were inactive on Rb.A. (B1). These results suggest that kinins act on at least four functional sites: B1 (Rb.A.), B2A (Rb.J.V.), B2B (G.P.I.) and BH. However, there is no clear evidence of a kinin receptor on rat mast cells and the release of histamine may simply be a non-receptor phenomenon. Our data also show that B2A and B2B receptor subtypes might simply be variations of the B2 receptor in different species.