A B S T R A C T To see whether antihistamines could prevent and reverse histamine-induced pulmonary edema and increased lung vascular permeability, we compared the effects of a 4-h intravenous infusion of 4 /Ag/kg per min histamine phosphate on pulmonary hemodynamics, lung lymph flow, lymph and plasma protein content, arterial blood gases, hematocrit, and lung water with the effects of an identical histamine infusion given during an infusion of diphenhydramine or metiamide on the same variables in unanesthetized sheep. Histamine caused lymph flow to increase from 6.0±0.5 to 27.0±5.5 (SEM) ml/h (P <0.05), lymph: plasma globulin concentration ratio to increase from 0.62±0.01 to 0.67±0.02 (P < 0.05), left atrial pressure to fall from 1+1 to -3±1 cm H20 (P < 0.05), and lung lymph clearance of eight protein fractions ranging from 36 to 96 A molecular radius to increase significantly. Histamine also caused increases in lung water, pulmonary vascular resistance, arterial Pco2, pH, and hematocrit, and decreases in cardiac output and arterial Po2. Diphenhydramine (3 mg/kg before histamine followed by 1.5 mg/ kg per h intravenous infusion) completely prevented the histamine effect on hematocrit, lung lymph flow, lymph protein clearance, and lung water content, and reduced histamine effects on arterial blood gases and pH. 6 mg/kg diphenhydramine given at the peak histamine response caused lymph flow and lymph: plasma protein concentration ratios to fall. Metiamide (10 mg/ Description of the preparation We used an unanesthetized, chronic sheep preparation described previously (1-4). Each animal was prepared by a series of three staged thoracotomies during which nonpulmonary contributions to a large lymph node in the posterior mediastinum (caudal mediastinal node) were resected;