Synthetic leukotrienes C4 and D4 (LTC and LTD) were found to possess potent coronary vasoconstrictor and cardiac depressant actions on isolated guinea-pig hearts. We therefore went further to investigate the possibility that endogenously released slow-reacting substance of anaphylaxis (SRS-A) might be responsible for the coronary vasoconstriction and negative inotropism in guinea-pig cardiac anaphylaxis. Results using time-course analysis as well as the specific SRS antagonist FPL 55712 have shown that SRS-A released during cardiac anaphylaxis was unlikely to be responsible for the early and most dramatic phase of coronary vasoconstriction that usually occurred at the 2nd min after antigen challenge, but could possibly be responsible for the latter and more prolonged phase occurring between the 6th and 14th min. This is because SRS-A release was found to peak at the 4th min after antigen challenge, 2 min after vasoconstriction had already peaked. Moreover, this early component of coronary vasoconstriction could not be blocked by FPL 55712, whereas the latter component was significantly reduced by the antagonist. The negative inotropism following cardiac anaphylaxis was also found to be significantly reduced by FPL 55712, thus suggesting SRS-A involvement. However, our experiments did not show whether the two actions were direct effects of SRS-A or whether contractility failure was a consequence of coronary vasoconstriction.