The biliary excretion of intravenously injected ouabain and the diffusion constant of the lateral mobility of hepatocyte plasma membrane proteins were examined in control (saline-treated) and spironolactone-treated Wistar male rats of different ages (4, 14 to 15 and 24 months old). The biliary excretion of ouabain progressively decreased with age in control rats, the first 10-min biliary recovery in 24-month-old animals being one-third that of the youngest rats (4-month-old). The oral administration of spironolactone for 4 days (10 mg per 100 gm body weight on the first day and 20 mg per 100 gm body weight for the successive 3 days) caused a marked increase in the biliary recovery of ouabain in all age groups. Similarly, the average lateral diffusion constant of hepatocyte plasma membrane proteins as measured by fluorescence recovery after photobleaching showed a linear decrease with age, as was previously observed with F-344 rats of both sexes. Markedly and significantly (30 to 40%) higher diffusion constants were observed in rats pretreated with spironolactone for all three age groups, compared with the respective control values of corresponding ages. The parallelism between ouabain excretion and protein diffusion (i.e., a decrease with age and an increase with spironolactone pretreatment) suggests that the lateral mobility of proteins in the hepatocyte plasma membrane is a candidate mechanism for regulating ouabain excretion through the liver into the bile, most probably by regulating the hepatic uptake process for ouabain.