Isolated hepatocytes were prepared from Wistar-derived male rats of different ages (4, 12 and 27-29 months) by the collagenase perfusion method. The hepatic uptake rate of taurocholate (TC) for the saturable fraction was calculated by subtracting the non-saturable fraction from the total hepatic uptake. The Vmax and the apparent affinity constant Km were computed for the saturable fraction by means of non-linear regression. The Vmax (nmol/mg protein/min, mean +/- SE) for young rats (N = 6) was 2.15 +/- 0.11, whereas in old rats (N = 4) the value was 50% lower (1.16 +/- 0.11, P less than 0.005). In contrast, Km (microM) values were not significantly different between young (25.88 +/- 1.90) and old (30.34 +/- 4.96) rats. There was a significant inverse linear relationship (r = 0.79; P less than 0.01) between the age of rats and the uptake velocity (nmol/mg/mg protein/min) at 1 microM of TC, suggesting a steady and almost linear decrease of TC uptake velocity with age. The rate of decrease per month (2.1%) was quite close to the value for ouabain uptake (2.8%) previously found by the authors. Furthermore, a marked linearity was observed between the average values for TC uptake rates for three age groups and corresponding lateral diffusion constants of hepatocyte plasma membrane proteins previously obtained by the authors using the fluorescence recovery after photobleaching method. The results support our previous proposal that protein mobility within the hepatocyte surface membrane may play at least a partial role in regulation of carrier-mediated hepatocyte uptake functions for various materials.