Rats with one kidney clamped (2K1C), both kidneys clamped (2K2C), unilaterally nephrectomized with remaining kidney clamped (1K1C), and normals, were studied using 99mTc mercaptoacetyltriglycine ([ 99mTc]MAG-3) and 131I orthoiodohippurate ([131I]OIH). Clearances of [99mTc]MAG-3 and [131I]OIH were performed after constricted rats became hypertensive. Clearances were repeated after i.v. Captopril. Clearances of [99mTc]MAG-3 and [131I]OIH in normals didn't change significantly after Captopril. Clearances of [99mTc]MAG-3 and [131I]OIH decreased insignificantly after Captopril in the 2K2C model. in the 2K1C group, normal kidney clearance increased ([99mTc]MAG-3 p less than 0.01 and [131I]OIH p less than 0.025) and clamped kidney clearance decreased after inhibition ([99mTc]MAG-3, p less than 0.01, [131I]OIH p less than 0.02). Clearances increased in the 1K1C group after Captopril ([99mTc]MAG-3 p less than 0.0025 and [131I]OIH, p less than 0.001). The ratio of [99mTc]MAG-3 to [131I]OIH before Captopril was 0.81 and 0.84 after Captopril. Changes in renal function after Captopril depend on the model of renovascular hypertension and possibly the dose administered. Technetium-99m MAG-3 clearance parallels [131I]orthoiodohippurate in renovascular hypertension.