The present in vitro microperfusion study examined whether chlorpropamide (CPM) has a direct effect on hydraulic conductivity (Lp x 10(-6) cm/atm.sec) and 14C-urea permeability (Pu x 10(-5) cm/sec) in the middle and distal inner medullary collecting duct (IMCD) obtained from acutely water-loaded Wistar rats and rats homozygous for diabetes insipidus (DI). CPM (10(-4) M) added to the bath fluid increased the Lp in the water-loaded Wistar rats from -0.05 +/- 0.13 to 6.25 +/- 0.74 (p less than 0.01) and in the DI rats from 0.05 +/- 0.01 to 5.95 +/- 0.84 (p less than 0.01), but had no effect when it was added to the perfusate. CPM stimulated Lp in a dose-dependent manner with the threshold effect at 10(-6) M. However, the addition of CPM (10(-4) M) to submaximal concentration of VP in the bath fluid did not increase the Lp. Furthermore, CPM was unable to block the inhibitory action of PGE2 on the vasopressin (VP)-stimulated Lp. On the contrary, PGE2 blocked the CPM-stimulated Lp. CPM (10(-4) M) in the peritubular fluid was able to cause a significant rise of the Pu from 13.5 +/- 0.8 to 17.3 +/- 1.0 reversibly, which represented 16% of maximum stimulated effect produced by 50 microU/ml of VP. Thus, pharmacological doses of CPM added to the peritubular side have a direct effect on terminal IMCD increasing water and urea permeability in the absence of VP, but this drug does not potentiate the VP-stimulated water transport in the IMCD. Our results were unable to confirm the hypothesis that CPM potentiates the VP-antidiuresis by the inhibition of PGE2 action in the rat IMCD.