The inhibitory effects of pituitary allografts on the prolactin (PRL)-secretory system are presumed to be consequences of the unabated release of PRL by the allografts. In the present studies we used pituitary allografts in the Golden Syrian hamster to address the following questions: (a) Do allografts of adult adenohypophysial tissue which elevate serum PRL levels decrease the concentration of PRL mRNA in the host’s adenohypophysis? (b) Is this effect shared by allografts of neonatal hypophysial tissue or neonatal muscle tissue which do not elevate serum PRL levels? (c) Do any of these types of allograft alter growth hormone mRNA in the host’s adenohypophysis? Prolactin mRNA concentration, but not growth hormone mRNA concentration, was decreased in the adenohypophyses in situ in the hosts bearing adult adenohypophysial allografts in which serum PRL levels were elevated. In contrast, serum PRL in hosts with neonatal hypophysial or muscle allografts were not elevated and PRL mRNA levels in the adenohypophysis in situ were not decreased when compared to the levels measured in hamsters with sham transplants. Prolactin mRNA levels in hosts with neonatal muscle allografts were not different from levels in hosts with neonatal hypophysial allografts but were increased when compared to the levels measured in hamsters with sham transplants. There were no differences in PRL concentration in the adenohypophyses in situ between any of the groups. Also, PRL concentrations in neonatal hypophysial allografts were similar to those in adult adenohypophysial allografts. To our knowledge these observations are the first demonstrating that short-loop feedback of PRL includes a decrease in PRL mRNA concentration. The observations also support the working hypothesis that PRL and not another pituitary factor exerts the negative feedb