Leukotriene D4 (1--20 micrograms/kg i.a.) administered to conscious spontaneously hypertensive rats (SHR) and WKY rats caused acute elevation of blood pressure in both groups, but only in SHR a prolonged hypotensive period followed the hypertensive event. SHR rats had tachycardia during the hypertensive phase and relative bradycardia during the hypotensive phase which was more pronounced and prolonged than in WKY rats. In SHR rats only, plasma epinephrine and norepinephrine were elevated (6- and 3-fold, respectively) at the peak of the hypertensive period. Pretreatment of SHR rats with indomethacin (5 mg/kg) potentiated the LTD4-induced pressor response and shortened the hypotensive-bradycardic effect of LTD4. This same biphasic, dose-dependent response to LTD4 (1--20 micrograms/kg i.v.) was present in pithed SHR rats. Therefore, a direct action of LTD4 on vascular smooth muscle and heart is suggested. In all WKY rats and some SHR rats, a bradycardic effect of LTD4 resulted from sinus bradycardia, whereas in pithed SHR rats impaired conduction varying from transient second degree atrioventricular block to complete heart block was observed. Electrocardiographic signs of ischemia were seen only in LTD4-injected, pithed SHR rats. These results suggest fundamental differences between SHR and WKY rats in regard to their sensitivity to lipoxygenase products.