The distribution pattern of rat liver parenchymal cells of different ploidy classes has been investigated with isolated liver cell preparations on four different time points of the day. The percentage distribution of different ploidy populations was found to fluctuate along the activity-rest cycle of the animals. The mononuclear tetraploid (MT) cell population showed the strongest time-dependent distribution pattern with a 10% variation around the 24 h mean, the variations in the binuclear diploid (BD) cells being somewhat less obvious, but clearly present. The mononuclear diploid cells and the binuclear tetraploid cells occurred in a virtually constant percentage. An inverse correlationship was noted between MT and BD cells, the increase in the number of MT cells being accompanied by a decrease in the number of BD cells. This interdependency in the presence of the two cell types can be related to their relationship in the process of formation.