The effects of PDN on VP-sensitive cAMP metabolism were examined in MCT and MAL microdissected from the rat kidney. VP-sensitive adenylate cyclase activity was significantly reduced (delta -46%; p less than 0.05) in MAL of PDN rats but, in sharp contrast, was significantly increased (delta +79%; p less than 0.02) in MCT of PDN rats compared to controls. cAMP phosphodiesterase activity was significantly increased in both MAL (delta +59%; p less than 0.005) and MCT (delta +79%; p less than 0.001) of PDN rats compared to controls. The increase in cAMP accumulation in MAL measured in response to VP in intact tubules did not differ between PDN and controls, whereas cAMP accumulation in response to VP was significantly higher (delta +127%; p less than 0.001) in MCT of PDN rats compared to controls. The present results would indicate that the observed in vivo resistance to the antidiuretic effect of VP that occurs in PDN is not due to an impairment in VP-sensitive cAMP accumulation in MCT, but would rather suggest that a defect exists at a cellular step subsequent to cAMP generation. In addition, our results illustrate that the extent and directionality of in situ accumulation of cAMP measured in intact tubules cannot always be predicted from rhe activities of enzymes controlling its synthesis and degradation (adenylate cyclase and cAMP phosphodiesterase), which are measured in vitro in disrupted tubules.