Glycerophosphoinositols are metabolites formed by a phosholipase A2 and a lysolipase specifically acting on membrane phosphoinositol lipids. High levels of these compounds characterize epithelial cells and fibroblasts transformed by ras and other cellular oncogenes. Here we have analyzed the glycerophosphoinositol levels in cells that are considered models of cell differentiation. Using rat hepatocytes at different stages of liver development we have shown that the glycerophosphoinositol basal levels of fetal cells were up to fourfold higher than in adult hepatocytes. No changes in glycerophosphoinositol were observed in regenerating rat liver, a model of differentiated cells proliferating in a synchronous manner, where only glycerophosphoinositol 4-phosphate increased by 80%. Similarly to fetal hepatocytes, a modest but significant increase (30%) in the levels of glycerophosphoinositols was observed in undifferentiated NG-108-15 cells as compared to the same cells induced to differentiate by cAMP. In a different neuronal cell line, PC12 cells, increased glycerophosphoinositol levels characterized the differentiated cells. Based on these observations we suggest that high glycerophosphoinositol levels characterize cellular phenomena associated with the activation of ras/mitogen-activated protein kinase pathways.