PIP: To study the production of antiaggregatory PGI2 (prostacyclin) the authors, observed 3 groups of women: 1) 34 women on combined OCs (oral contraceptives) for a period of 4 months--7 years; 2) 11 women starting contraception with progestogen only pills, or with low-dose estrogen pills; they were followed for a period of 3 months; and, 3) 24 women who had never been on OC. Blood samples were taken every month and examined. Results showed that: 1) the prolonged use of combined OCs was associated with decreased production of PG12, especially in relation to the length of OC treatment; 2) PG12 production was not affected by contraception with progestogens only; and, 3) no modifications were found in users of low-dose estrogen pills. The principal biological funcion of PG12 is to inhibit platelet aggregation, and to keep a proper dilatation of the blood vessels. Thus, suppression of PGI2 can be associated with an increased risk of venous and arterial thromboembolism in OC users. While the risk of thromboembolism may be decreased with low-dose estrogen treatment, risk of arterial thromoboembolism is not decreased, since OCs exercise a lesser influence on areterial thrombosis.