Abstract: Objective 　To study the mechanism of prolactinomas induced by estrogen in rats. Methods 　Adult Wistar rats were divided into 2 groups at random. Each rat in cont rol group ( n = 16) were subscutaneously implanted with a blank implant . Rats in estrogen group ( n = 16) were implanted with est rogen-containing implants. The animals were executed af ter 2, 4, 6, 8 weeks. Serum prolactin ( PRL ) levels were measured by RIA method. Pit-1 mRNA level in pituitary tissue were measured by RT-PCR method. Results 　Pit-1 mRNA levels were different among 2, 4, 6, 8 weeks and increased along with 2, 4, 6, 8 weeks. There was a positive relation between Pit-1 mRNA levels and serum PRL levels (γ= 0. 964, P = 0. 01) . There was also a positive relation between Pit-1 mRNA levels and pituitary weight s (γ= 0. 913, P = 0.01) . Conclusion 　Estrogen has a stimulating effect on the expression of Pit-1 gene, which plays an important role on est rogen induced prolactinoma.