1. Platelet activation in vivo occurs in healthy pregnancy and is more pronounced in pre-eclampsia.

2. This study has investigated: (i) the inhibitory potency of the nitric oxide donors 3-morpholinosydnonimine and sodium nitroprusside, on the platelet release reaction in vitro in non-pregnant, healthy pregnant and pre-eclamptic women; (ii) the concentration of cyclic GMP during incubation of washed platelets with sodium nitroprusside in a separate group of non-pregnant, healthy pregnant and pre-eclamptic women.

3. The half-maximal inhibitory concentration of sodium nitroprusside, in the presence of a phosphodiesterase inhibitor, for inhibition of the platelet release reaction was lower in the pre-eclamptic subjects than in the non-pregnant subjects (P < 0.05).

4. Several of the pre-eclamptic women were studied again postnatally. The half-maximal inhibitory concentrations of sodium nitroprusside and 3-morpholinosydnonimine were higher in the postnatal than in the antenatal sample (P < 0.02).

5. Peak platelet cyclic GMP responses to sodium nitroprusside were significantly higher in the pre-eclamptic women than in the healthy pregnant and non-pregnant women.

6. These results suggest that platelets are more sensitive to the inhibitory effects of nitric oxide donors in pre-eclampsia.

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