Platelet function was studied in 30 pregnant women: 14 normotensive (C), and 16 affected by pregnancy-induced hypertension (PIH). Platelet aggregometry (PA) on platelet-rich plasma according to Born was compared with the new PFA-100 System (Dade International Inc, Miami, USA). This device evaluates platelet function (expressed in seconds as closure time, CT) in anticoagulated whole blood ex vivo at high shear rates. PA (expressed as percentage of light transmission) and CT were measured at baseline and after incubation with L-Arginine (L-Arg). MANOVA for repeated measures showed that L-Arg incubation significantly decreased PA (F=7.2, P < 0.05) and increased CT (F=6.05, P < 0.05) in the whole population of pregnant women. Moreover, we analysed separately both parameters in C and in PIH subjects. No differences in PA were found in both groups, neither at baseline nor after L-Arginine incubation. In contrast, CT was significantly longer in PIH in comparison to C before (95.9 s vs. 84 s, P < 0.05) as well after (115 s vs. 92 s, P < 0.05) L-Arginine incubation. Data from PFA-100 confirm our previous reports that during pregnancy the L-Arginine: Nitric Oxide pathway regulates platelet function. In hypertensive patients a significant decrease in platelet function was found by using the PFA-100 system.