The relationship between cell proliferation and inositol lipid turnover has been studied by comparing the steady state of inositol derivative metabolism in quiescent and regenerating rat hepatocytes isolated at 4 h (GI phase of first cell cycle) and 24 h (onset of M phase) after partial hepatectomy. The effect of two hormones able to regulate hepatic regeneration, insulin and vasopressin, has been considered, and the results can be summarized as follows: (i) at 4 h after partial hepatectomy, the precursor incorporation into inositol polyphosphates and the particulate phospholipase C activity increase with respect to quiescent hepatocytes, whereas the content of II,4,5P3 does not change, suggesting an increased turnover of this molecule in this step of cell cycle priming; (ii) 24 h after partial hepatectomy, the radioactivity linked to IP, and I P, , as well as soluble and particulate phospholipase C activity, and IP, content increase, suggesting the presence, at the onset of M phase, of second messenger accumulation; (iii) only 24 h after partial hepatectomy, the inositol derivative metabolism is affectcd by vasopressin; and (iv) insulin exerts a modulatory role on inositol polyphosphate production without involving membrane-bound PLC activity or phosphoino5itide hydrolysis. These data suggest that inositol-derived signal molecules are associated with hepatic regeneration; moreover, the metabolic pathway of such compounds seems to be regulated so that only specific inositol phosphates are present in each step of the cell cycle.