1. The effects of volume contraction and indomethacin on renin response were examined in spontaneously hypertensive rats and Wistar-Kyoto normotensive rats. Volume contraction was induced by frusemide or by salt-restricted diet combined with frusemide administration. 2. Plasma renin levels were not altered by either procedure in spontaneously hypertensive rats (5.2 ± 0.8 versus 5.6 ± 0.9 ng h−1 ml−1). Normotensive rats responded to volume contraction with a sharp increase in plasma renin activity (13.1 ± 1.2 to 23.3 ± 1.1 ng h−1 ml−1). 3. Intraperitoneal injection of indomethacin for 1 week did not alter basal renin levels in either group. In contrast, indomethacin pretreatment caused renin to rise in response to frusemide in spontaneously hypertensive rats (4.7 ± 0.8 to 27.1 ± 1.8 ng h−1 ml−1). 4. These findings suggest that a prostaglandin normally inhibits the renin response of spontaneously hypertensive rats to frusemide-induced volume contraction. Inhibition of prostaglandin synthesis allows volume contraction to stimulate renin release.