The relationship between the level of estradiol (E2) and content of apolipoprotein AI and B in female rat serum during sexual cycle or after ovarectomy and injection of E2 was observed. The results indicate that there were cyclical changes of E2 and apolipoprotein AI content in female rat serum during sexual cycle, the tendency of changes of E2 and apolipoprotein AI was idencal, the highest values of them were obeserved at the period of proestrum. In ovariectomized rats the contents of E2, apolipoprotein AI in serum were significantly lower than those in each period of sexual cycle (P<0. 01 ). In ovariectomized and injected E2 group, the content of E2 and apolipoprotein AI were significantly higher than those in ovariectomized rats (P < 0. 01 ). The content of apolipoprotein B in serum remained unchanged either in sexuale cycle or in ovariectomized and injected E2group. This suggests that E2 may have a role to increase the content of apolipoprotein AI in serum.