In male Wistar rats weighing 290 to 343 g the left kidney was removed (n = 24) except in those belonging to the sham group (n = 6). One week afterwards, the poles of the contralateral organ were also removed surgically in 12 rats. Two groups of rats subjected to partial nephrectomy were obtained: G1, uninephrectomized, residual kidney mass 0.50 (n = 12); G2 uninephrectomized and subjected to the surgical removal of the poles of the contralateral organ; residual kidney mass 0.333 (n = 12). The G1 group was subdivided in G1C (n = 6) followed for 75 days without any treatment and G1V followed for 45 days without any treatment, receiving between 45 and 75 days verapamil, once daily: 0.2 microgram/g s.c. G2 was also similarly subdivided and treated. Every 15 days serum urea and creatinine were evaluated. On the 75th day inulin clearance was performed in all the animals. Under pentobarbital anesthesia, polyethylene catheters were placed in a carotid artery, jugular vein, urinary bladder and trachea. After the completion of surgery a prime dose of inulin (30 mg.100 g weight-1) was administered. Subsequently a constant dose of 0.5 g.min-1. 100 g weight-1 of inulin in isotonic saline was infused throughout the experiment by a constant infusion pump. Following a stabilization period of 45 minutes, two consecutive clearance periods of 30 minutes were evaluated in each animal. Urinary samples for the evaluation of inulin were collected from the urinary bladder throughout each period and arterial blood samples for determination of inulin were taken at the middle points of each period.(ABSTRACT TRUNCATED AT 250 WORDS)