The cardiovascular actions of two pyrethroids, deltamethrin and cismethrin, were assessed using the pithed rat, the isolated working heart, and perfused mesentery preparations. Deltamethrin but not cismethrin, increased mean arterial pressure and differential pressure in the pithed rat, and the aortic output and mean systolic aortic pressure in the working heart. Reserpine pretreatment reduced, but did not abolish, the pressor response to deltamethrin in the pithed rat and working heart, but did not reduce the increase in aortic output following deltamethrin in the working heart. In the perfused mesentery, deltamethrin did not modify the action of exogenous noradrenaline, but increased the response to 10 Hz stimulation. It is concluded that the cardiovascular effects of deltamethrin are due to both increased catecholamine release in peripheral vascular beds, and to a direct positive inotropic effect on the heart. The results also provide a distinction between the action of the two pyrethroids in terms of their cardiovascular activity.