Three α2u-globulin products were identified on two-dimensional gel electrophoretograms of the [35S]methionine-labeled products formed in response to total hepatic RNA by the reticulocyte lysate translational assay. These products, arbitrarily designated A, B, and C, were quantified by videoimaging of two-dimensional electrophoretograms. At both 65 and 85 days, they represent approximately 13%, 10%, and 77%, respectively, of the total translational products coding for α2u-globulin. Hypophysectomy at either 30 or 55 days resulted in a marked decrease in all three products when measured at 60 and 85 days. Products A and B were not induced in animals hypophysectomized (hypox) at 30 days and subsequently reinjected for 10 days beginning on day 50 with GH alone; with T4, corticosterone, and dihydrotestosterone (three hormones); or with a four-hormone combination. Product C was significantly induced by the four-hormone combination. In animals hypox at 55 days and reinjected for 10 days beginning on day 75, GH alone and the three-hormone combination failed to increase product A. In contrast to the observations in animals hypox at 30 days, the four-hormone combination induced a significant increase in products A arid B in animals hypox at 55 days, and product C was induced by the three-hormone as well as the four-hormone combination. Measurement of total α2u-globulin mRNA by cDNA hybridization indicated that animals hypox at 30 days achieved only 33.8% of age-matched normal levels, whereas animals hypox at 55 days achieved 69% (P < 0.05) when injected with all four hormones. Furthermore, the three-hormone combination increased total α2u-globulin mRNA in animals hypox at 55 days, but not in animals hypox at 30 days. The observations on the individual α2u-globulin products suggest that endocrine regulation of individual α2u sequences is not identical. The different magnitude of response observed in the animals hypox at different ages suggests that exposure of the liver to a pituitarydependent factor between 30 and 55 days modifies the ability of the liver to regulate α2u-globulin mRNA levels.