Juvenile visceral steatosis (JVS) mice are associated with systemic carnitine deficiency (Kuwajima, et al., 1991). In order to investigate the cause of this deficiency, we compared fibroblast carnitine transport activities in normal mice and JVS mice. The kinetic analysis showed that in formal fibroblasts, the Km and Vmax values for saturable uptake was 15.6 microM and 2.56 pmol/min/mg protein, respectively. In JVS fibroblasts, however, saturable uptake was not observed. There was no great difference in the linear component of uptake between normal and JVS fibroblasts. At the physiological concentration (50 microM) of carnitine, the fibroblast carnitine transport activity in JVS mice was decreased to 18% of that in normal mice. Thus there is hardly any carnitine transport activity in the fibroblasts of JVS mice, indicating that the JVS mouse can be regarded as an animal model of primary carnitine deficiency.