Distribution of nuclear ploidy in female mouse hepatocytes was measured cytofluorimetrically using ethidium-bromide-stained hepatocyte nuclei liberated by in situ collagenase perfusion of the liver via the portal vein. After iv administration of lead acetate or an ip general anaesthetic (Valium and Hypnorm), rapid shifts of 8N and 4N nuclei to lower ploidy levels were recorded. It was possible to block the ploidy changes with ip colchicine, although no blocked metaphases were observed histologically. These observations were consistent with an earlier report on changes in nuclear ploidy induced by carbon tetrachloride (Steele et al., 1981b). It is speculated that these changes in ploidy may represent an early response of the hepatocytes to stimulation--an aspect of liver behaviour of which we were not previously aware.