Studies were carried out in conscious, chronically-cannulated, spontaneously hypertensive (SH) rats to determine the comparative cardiovascular actions of a series of diuretic compounds. The effects of three oral doses of hydrochlorothiazide, trichlormethiazide, chlorthalidone, metolazone, spironolactone, triamterene, amiloride, furosemide, ethacrynic acid, MK-447, hydrochlorothiazide + amiloride, and MK-447 + amiloride on blood pressure and heart rate were monitored continuously for four hr and again 24 hr post-dose. Thiazides, thiazide derivatives and antikaliuretic diuretics exerted little or no antihypertensive effects, while the loop-acting diuretics (except ethacrynic acid) and combinations markedly reduced blood pressure. Antihypertensive responses to furosemide, MK-447 and the diuretic combinations were associated with either stable heart rate or paradoxical bradycardia, but not compensatory tachycardia. Heart rate remained relatively unaltered by individual doses of the remaining compounds.