We studied the role of sodium (Na) in renin release from isolated afferent arterioles incubated in Krebs–Ringer bicarbonate solutions with the Na concentrations adjusted to 145, 135, 125, 110 and 95mmol/l. The arterioles were incubated for three consecutive periods, with the sequence of incubations randomized. Renin release increased by 119 and 275% in the media with Na concentrations of 135 and 125 mmol/l, respectively, as compared with rhe renin released from arterioles incubated in the medium with Na 145 mmol/l. When the Na concentrations of the incubation media were further lowered to 110 mmol/l and 95 mmol/l, renin release continued by 118 and 216%, respectively, in comparison with that in the medium having an Na concentration of 125 mmol/l. Regression analysis showed that renin release increased by 98% for each 10 mmol/l decrement of Na concentration. When the osmolarity of the incubation medium was held constant, varying the Na concentrations did not alter the renin secretion. This suggests that renin was released from the arterioles in response to the osmotic pressure exerted by Na rather than as an ionic effect. Renin release was suppressed by the osmotic effects of sucrose and urea, although the suppressive effect of urea was weaker.