Because, in sheep, histamine-induced increased lung vascular permeability is prevented by diphenhydramine, we tested the effects of diphenhydramine on the sheep lung vascular response to endotoxin. We infused E. coli endotoxin (0.40-1.00 micrograms/kg) with and without diphenhydramine (3.0 mg/kg bolus + 1.5 mg . kg-1 . h-1) in the same unanesthetized sheep while measuring pulmonary arterial (Ppa) and left atrial (Pla) pressures, lung lymph flow (Qlym) and lymph (L) and plasma (P) protein concentrations. Endotoxin caused pulmonary hypertension soon after infusion (base-line Ppa = 22 +/- 3 (SE) cmH2O; after endotoxin Ppa = 40 +/- 2; P less than 0.05, n = 6) and after several hours an increase in permeability reflected in high flow of protein-rich lymph (base-line; Qlym = 7.5 +/- 1.4 (SE) ml/h, L/P protein concentration = 0.60 +/- 0.02: after endotoxin; Qlym = 21.4 +/- 3.1, P less than 0.05; L/P = 0.66 +/- 0.03, P less than 0.05). In the presence of diphenhydramine, endotoxin caused identical pressure changes but Qlym was lower during the period of increased permeability (16.7 +/- 3.0 (SE) ml/h, P less than 0.05 compared to endotoxin alone) and L/P protein concentration was similar (0.68 +/- 0.04, P = NS). We conclude that endogenous histamine may be partly responsible for the increase in lung vascular permeability after endotoxemia, but that histamine probably is not the sole mediator of the permeability change.