Platelet-activating factor-acetylhydrolase (PAF-AH), the enzyme that inactivates PAF, is decreased in the plasma of both rabbits and humans during the latter stages of pregnancy. The activity of the enzyme was decreased in rats by the administration of 1,3,5(10)-estratrien-17 alpha-ethynyl-3,17 beta-diol (17 alpha-ethynylestradiol) and increased by dexamethasone treatment. In the present study, we have further defined the hormonal regulation of PAF-AH levels in plasma of adult and juvenile rats. Estrone (E1), 17 beta-estradiol (E2), estriol (E3), and various progestins [4-pregnen-6 alpha-methyl-17 alpha-ol-3,20-dione (medroxyprogesterone), 4-estren-17 alpha-ethynyl-17 beta-ol-3-one (norethindrone), and 5(10)-estren-17 alpha-ethynyl-17 beta-ol-3-one (norethynodrel)] were administered to adult rats, and the plasma PAF-AH activities were assayed. E1, E2, and E3 administration to adult female rats lowered the plasma PAF-AH activity, E3 being the most effective. After administration of these estrogens the activity returned to the preinjection level within 4 days. The administration of medroxyprogesterone resulted in a 2-fold increase in plasma PAF-AH activity in adult female rats, and the activity remained elevated for up to 30 days. When adult male rats were treated with similar doses only a 20% increase was observed and the PAF-AH activity returned to control values by day 10. In contrast, norethindrone and norethynodrel administration resulted in a reduction of the enzyme activity in adult female rats. The plasma PAF-AH activity in juvenile male and female rats (3 weeks of age, 45-60 g body wt) was two times higher than that in adult rats of both sexes and spontaneously decreased up to the time of puberty. When juvenile male or female rats were injected with either 17 alpha-ethynylestradiol or medroxyprogesterone, a minimal change in PAF-AH activity was observed. In contrast, when dexamethasone was administered to juvenile male and female rats the plasma PAF-AH activity increased in a manner similar to adult animals. It is suggested that estrogens cause a decrease and medroxyprogesterone an increase in plasma PAF-AH activity. It is suggested that the different responsiveness to medroxyprogesterone between adult female and male rats and juvenile animals may depend on the concentration of the hormone receptor in the tissue responsible for the synthesis of PAF-AH. The decrease in PAF-AH activity after the administration of norethindrone and norethynodrel may be due to the known estrogenic activity of these steroids.