Apoptosis is a form of cell death involved in the regulation of cell number in various organs and tumors. Quantitative determination of cell loss through apoptosis in histological sections requires, in addition to counts of apoptotic cells, information on the duration of the histologically visible stages of apoptosis. Here we describe a method to determine the duration of apoptosis in (i) normal and (ii) putative preneoplastic tissue of the liver. (i) Female rats were treated with high doses of the hepatomitogen cyproterone acetate (CPA) to induce liver hyperplasia. After stopping CPA treatment, the hyperplasia partially regressed and excessive hepatocytes were eliminated by apoptosis. CPA was given to block the initiation of apoptosis, and thereafter the time course of elimination of apoptotic cell residues (apoptotic bodies, ABs) from the liver was studied. The mean duration of the histological stages of apoptosis was found to be approximately 3 h. (ii) Phenotypically altered cell foci in rat liver were produced by a single dose of N-nitrosomorpholine and subsequent promotion for 39 weeks with phenobarbital (PB). PB was withdrawn to stop foci growth and to stimulate apoptosis. Then rats were retreated with PB to block initiation of apoptosis in foci. The results indicate that the majority of apoptotic bodies in foci disappeared within 4 h after PB, suggesting that the stages of apoptosis are as short in foci as in normal liver. Finally a simple formula is given to calculate the cell loss rate by apoptosis. The method presented may provide data for quantitative cancer risk assessment from mathematical models of carcinogenesis.