We investigated whether hypermagnesemia alleviates hypoxic or group B streptococcal (GBS) pulmonary hypertension (PH). Hypoxic PH was induced and maintained in 14 lambs by continuous ventilation with 12% oxygen. GBS PH was induced and maintained in 16 lambs by the continuous infusion of 5-10 x 10(8) colony-forming units.kg-1.h-1 of GBS. After the onset of PH, lambs were randomized to receive either magnesium sulfate (MgSO4, intermittent boluses of 0.38 mmol/kg, with a continuous infusion of 0.15 mmol.kg-1.h-1) or a similar volume of normal saline. Hypermagnesemia lowered pulmonary arterial pressure (PAP) and delayed the fall in systemic arterial pressure and stroke volume index seen in the control animals (each P < 0.05). At a serum magnesium concentration ([Mg]) of 2.75 +/- 0.25 mmol/l, PAP was 27 +/- 3 compared with 40 +/- 4 Torr in the control animals ([Mg] = 0.87 +/- 0.06 mmol/l; P < 0.05). In the GBS PH trial, hypermagnesemia prevented the continued increase in PAP seen in the control animals. At [Mg] = 2.15 +/- 0.07 mmol/l, PAP fell 2 +/- 1 Torr from prerandomization values, whereas it rose 4 +/- 2 Torr in the control animals ([Mg] = 0.59 +/- 0.07 mmol/l; P < 0.05). However, during the same time the systemic arterial pressure fell further in the magnesium-treated animals (-19 +/- 1 vs. -2 +/- 5 Torr). MgSO4 attenuates PH in both models but may cause systemic hypotension in sepsis.