Nitric oxide (NO) production by cells of the regenerating liver was estimated from the amount of nitrite accumulated during 24 h in the culture media of hepatocytes, Kupffer cells and sinusoidal endothelial cells isolated at different times after partial hepatectomy (PHE). The time course of NO production was compared with the course of the proliferating activity of the same cells. During the time when liver cells pass through their first cell cycles, hepatocytes were the main producers of NO in the liver. The time-dependent changes of their NO production corresponded to those obtained with the whole liver and were inversely correlated with the DNA-synthesizing activity. The NO production by Kupffer and endothelial cells followed that by hepatocytes in this order; the time displacement between them corresponded to the schedule of their proliferating activity. The NO synthesis in non-parenchymal cells fluctuated in a similar way as in parenchymal cells and was minimal when DNA synthesis was manifest.