The uptake and release of carnitine in primary-cultured guinea pig hepatocytes were investigated. Hepatocyte monolayers were incubated for 1-24 h in medium containing 0.05 or 0.1mM L-carnitine as an initial concentration. The contents of acid-soluble total and short-chain carnitine in cells incubatsed with 0.1mM carnitine increased with time, whereas the carnitine content of cells incubated with 0.05mM carnitine remained constant after reaching a maximum at 10 h. Free and total carnitine in cells preincubated with 0.5mM carnitine decreased rapidly during incubation in medium without carnitine, whereas the concentration of carnitine in the medium increased. Preloading with fatty acids did not affect the uptake of acid-soluble total carnitine, but decreased the content of free carnitine.