Single dose pharmacokinetics of 75 mg aspirin was investigated in two groups of ten women with clinically normal pregnancies. Eleven non-pregnant subjects in the same age were controls. In group A, gestational age was 27-30 completed weeks, and in group B, 36-39 weeks. Venous blood samples were taken before and up to 240 minutes after the intake of the aspirin. Liquid chromatographic assays for acetylsalicylic acid (ASA) and its metabolite, salicylic acid (SA), was performed. The pharmacokinetics of ASA and SA were similar in group A and B but pregnant subjects had a slower uptake and a lower peak level than controls. The late elimination phase for their compound did not differ between the groups. Nine pregnant women with normal pregnancies had their bleeding time measured by a modified Ivy technique using a Simplate II device before, at the end of, and two weeks after daily administration of 75 mg ASA for two weeks. All had a normal bleeding time before and two weeks after the end of the medication. Eight of the nine subjects had an increased bleeding time by Ivy tests, (p < 0.01) whereas the bleeding time assessed by Duke's method was within normal limits. These studies suggest that during pregnancy changes of the uptake rate and distribution volume modulate the pharmacokinetics of ASA and that this drug given in low dosage to gravidae marginally alters their platelet function.