Previously, we reported that natriuretic and diuretic response to dopamine is diminished in old Fischer 344 rats, which is due to higher basal protein kinase C (PKC) activity and hyperphosphorylation of Na-K-ATPase in the proximal tubules (PTs) of old rats. The present study was conducted to determine whether higher PKC activity could be due to altered expression of some of the PKC isoforms in the superficial cortex (rich in PTs) of old rats. Fluorimetric measurement showed almost twofold increase in the PKC activities in homogenates and membranes of old (24 mo) compared with adult (6 mo) rats. Interestingly, in the basal state PKC-betaI was overexpressed in the membranes, whereas PKC-delta expression was increased in the cytosol of old compared with adult rats. Treatment of the cortical slices with either SKF-38393, a D1-like agonist, or PDBu, a direct activator of PKC, caused translocation of PKC-betaI from cytosol to membranes in adult but not in old rats. Both of these drugs caused translocation of PKC-delta from membranes to cytosol in adult but not in old rats. These drugs had no effect on translocation of PKC-zeta in both adult and old rats. Both PKC-betaI and -delta co-immunoprecipitated with alpha1-subunit of Na-K-ATPase in adult and old rats. These observations suggest that both SKF-38393 and PDBu differentially regulate PKC-betaI and -delta in adult but not in old rats. Also, PKC-betaI and -delta seem to interact with Na-K-ATPase in these animals. The overexpression of both PKC-betaI and -delta in old rats could be responsible for a higher basal PKC activity, which causes the hyperphosphorylation of Na-K-ATPase and contributes to the diminished inhibition of Na-K-ATPase activity by dopamine in old rats.