The present study compares the effects of Ro 31-6930, a novel potassium channel opener, with those of cromakalim and nitrendipine on blood pressure and other haemodynamic parameters. In conscious, spontaneously hypertensive rats (SHR) the oral dose of Ro 31-6930 for lowering blood pressure was 10 times lower than that of cromakalim and some 100 times lower than that of nitrendipine. In addition, the duration of antihypertensive activity of Ro 31-6930 was longer than that of cromakalim or nitrendipine. The tachycardia evoked by Ro 31-6930 and cromakalim was of shorter duration than the antihypertensive effect of either agent. In a repeat, once daily dosing experiment no tolerance was observed to the antihypertensive effect of Ro 31-6930 over a 22-day period. In conscious normotensive cats Ro 31-6930 was 10 times more potent than cromakalim and 1,000 times more potent than nitrendipine in reducing blood pressure. The duration of hypotensive activity was in excess of 5 h for each agent. In anaesthetised dogs all three agents reduced mean arterial pressure (MAP) and total peripheral resistance (TPR), while increasing cardiac output (CO) via a rise in stroke volume (SV). Both Ro 31-6930 and cromakalim significantly reduced femoral (FVR) and mesenteric vascular resistances (MVR), while only cromakalim reduced renal vascular resistance (RVR). Ro 31-6930 is a potent new antihypertensive agent that compares favourably with cromakalim and nitrendipine.