1. Platelet behaviour in vitro in relation to cyclic AMP was studied longitudinally during pregnancy and in the same women when they were not pregnant. Subjects comprised a group of healthy primigravidae and a group of women deemed at risk of pre-eclampsia, on the basis of a previous history of the condition.

2. There was a progressive decline during pregnancy in sensitivity of platelets to inhibition of the arachidonic acid-induced release reaction by agents which act via cyclic AMP. This effect was maximum at 36 weeks' gestation.

3. Basal platelet cyclic AMP levels, and those in the presence of a phosphodiesterase inhibitor, did not change throughout the period of the study.

4. By contrast, platelet cyclic AMP accumulation in response to a variety of adenylate cyclase stimulators was reduced from early pregnancy, throughout the gestational period, compared with post-natally. This effect was noted when platelets were incubated with prostaglandins acting via different surface receptors or with forskolin and was most marked on co-incubation with a phosphodiesterase inhibitor.

5. Compared with healthy women, platelets from women with a previous history of pre-eclampsia tended to accumulate less cyclic AMP in response to adenylate cyclase stimulators. This was the case both during pregnancy and post-natally. Further investigation of adenylate cyclase activity in platelets in relation to pre-eclampsia is required.

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