A coculture system for in vitro proliferation of rat primary hepatocytes with feeder cells was investigated with the view of constructing a hybrid artificial liver module using human hepatocytes. A mouse cell line, STO, was apparently effective compared with other kinds of feeder cells such as FLS5 and MRC5 in increasing the total cell number in the coculture of rat primary hepatocytes. The parenchymal hepatocyte, which are polygonal cells, spread well as the cultivation progressed. Monitoring of parenchymal hepatocyte colonies under a microscope showed that the area of each colony increased as the single culture and cocultures progressed. The adhesion area per hepatocyte increased little during the coculture with STO cells, while the adhesion area increased markedly in the single culture of hepatocytes. The density of hepatocytes increased more than two times during the coculture for 5 d, while it decreased during the single culture. The production rate of urea by hepatocytes markedly increased during the coculture, while the rate in the single culture was lower than the coculture and increased only slightly. The specific rate of urea production by hepatocytes in the coculture did not decrease even as the hepatocytes grew. Consequently, rat primary hepatocytes could proliferate in vitro by coculture with STO cells without a decrease in urea production activity.