Leukotriene D4 (LTD4, 0.8-4 micrograms/kg i.a.) increased blood pressure and heart rate of conscious SHR and WKY rats. LTD4, 20 micrograms/kg (i.a.), caused a short lasting increase in blood pressure and heart rate in WKY. In SHR, LTD4, 20 micrograms/kg (i.a.) caused a triphasic effect: hypotension and bradycardia (0.5-1 min), hypertension and tachycardia (2-5 min) and prolonged hypotension and bradycardia (5-180 min). Leukopenia was observed after LTD4, 20 micrograms/kg in both groups of rats but platelets count was unaffected. These results show that SHR are mor sensitive than WKY to the hypotension-bradycardia effect of LTD4.