The intrarenal effect of diazoxide on renin secretion was examined in the isolated perfused rat kidney. Mean renin secretion measured at 2 min intervals during the continuous infusion of diazoxide for 6 min was consistently higher than the corresponding control values, although this was significantly different at the end of the infusion only. Since renal perfusion pressure during diazoxide infusion and control studies decreased to a similar extent, it is suggested that diazoxide stimulates renin secretion by a direct effect on the juxtaglomerular cell.