In an attempt to understand the nature of cytoplasmic and nuclear enlargement of liver cells designated as megalocytosis that results from chronic poisoning by lasiocarpine, a pyrrolizidine alkaloid, certain functional aspects of these cells were investigated together with the study of their morphology. The RNA polymerase activity of the megalocyte nuclei was essentially comparable to the activity observed in normal liver cells. Further, the inducibility of tryptophan pyrrolase activity by hydrocortisone, in the livers of rats treated chronically with lasiocarpine is an indication that translational mechanisms are intact. However, the increased uptake of 3H-thymidine by megalocytes, in the absence of observable mitotic activity, suggests that these cells are in the process of hypertrophy. It is concluded that the megalocytes are functionally normal cells, except that they are in the process of cellular hypertrophy and are incapable of division due to potent antimitotic action of the pyrrolizidine alkaloids.