We have used a combination of a sensitive immunocytochemical stain for intracellular albumin, and Hoechst 33258 dye for identification of parental nuclei to investigate the time-course of extinction, reexpression, and activation of albumin production in fusion products of 1s (hyperdiploid) or 2s (hypertetradiploid) rat hepatoma cells with mouse fibroblasts (L cells or embryonic cells). In all combinations, the initial event is extinction of albumin production. Extinction occurs immediately after fusion when the mouse fibroblast is a normal embryonic (senescent?) cell. In the case of an L cell, rat albumin is synthesized and secreted during the first 12 h after fusion; no production of mouse albumin occurs. Thereafter, albumin production ceases. 8-12 d after fusion, young hybrid colonies are found to resume the synthesis of rat albumin (reexpression), and several days later the production of mouse albumin begins (activation). The patterns of reexpression and activation indicate (a) that chromosome loss is not necessary for either event to occur and (b) that the cells active in the synthesis of mouse albumin are a subpopulation of those cells already engaged in the production of rat albumin. We conclude that (a) extinction is mediated by diffusible factor(s) from the L-cell parent that act in the hepatoma nucleus to prevent the formation of new albumin messenger RNA; (b) reexpression and activation are gene dosage- dependent but extinction is not; and (c) previously active genes are more rapidly expressed than previously silent ones.