OBJECTIVE: To evaluate nitric oxide (NO) production in patients with pregnancy-induced hypertension or preeclampsia and in controls. METHODS: Four groups of pregnant women were included: 17 patients with pregnancy-induced hypertension, ten with mild or moderate preeclampsia, 17 with severe preeclampsia, and 44 normotensive women matched for weeks of gestation at blood sampling with the cases. Plasma levels of L-citrulline and L-arginine were measured by using high-performance liquid chromatography. RESULTS: The mean plasma levels of L-citrulline and the ratio of L-citrulline to L-arginine, which reflects NO production, were higher in women with severe preeclampsia than in controls, patients with pregnancy-induced hypertension, and patients with mild or moderate preeclampsia. CONCLUSION: Nitric oxide production is enhanced in severe preeclampsia, possibly as a compensatory phenomenon for the increased synthesis and release of vasoconstrictors and platelet-aggregating agents.