The antiestrogen toremifene treatment results in regression or stabilization of dimethylbenz(a)anthracene(DMBA)-induced rat mammary tumors by reducing the mitotic activity and possibly by modifying the abundant spontaneous apoptosis seen in these tumors. A comparative study on the modes of cell loss was performed in untreated DMBA tumors and in tumors treated with toremifene and ovariectomy. Ovariectomy results in massive apoptotic cell death throughout the tumors with apoptotic cells exfoliating into the glandular lumina. Apoptosis accounts for most of the cell death also in the untreated tumors. The distinctly different morphology of the apoptotic cells in the untreated tumors (condensed apoptosis) and tumors treated with ovariectomy (blebbing apoptosis) may reflect the different type or schedule of the apoptotic process. Both morphologies of apoptotic cells were seen in the toremifene-treated tumors. Blebbing apoptosis is not easily examined in paraffin-embedded material but is best detectable in plastic sections and by electron microscopy.