Sphingomyelin signal transduction pathway regulates cell cycle through a number of lipid second messengers, which stimulate cell proliferation (sphingosine-1-phosphate), initiate growth arrest or induce apoptosis (sphingosine, ceramide). To asses the functioning of sphingomyelin pathway during liver regeneration after partial hepatectomy in rat (PH) we measured the content of sphingomyelin (SM), ceramide (CER), sphingosine (SPH), sphingosine-1-phosphate (S1P), the activity of neutral Mg(2+)-dependent and acidic sphingomyelinases and ceramidases, in the remnant liver lobes during the first 24h after PH in rat. The activity of acidic ceramidase was highest at 4th hour after PH, whereas the activity of neutral ceramidases peaked at 12th hour after the operation. At these time points the activity Mg(2+)-dependent sphingomyelinase was also elevated, together with the content of SPH, S1P and the ratio of S1P to CER. The activity of acidic sphingomyelinase increased gradually from 4th to 24th hour after the operation. This was accompanied by significant increase in the content of ceramide between 4th and 24th hour and reduction in the content of S1P and S1P to CER ratio. It is concluded that partial hepatectomy induces production of the pro-mitogenic intermediates of sphingomyelin signaling pathway during the first 12h of liver regeneration in rat.