The purpose of this study was to test the hypothesis that chlorpheniramine (CP), and H1 blocker, can cause bronchodilatation if administered intravenously (iv) and in higher doses than those currently prescribed. In 10 subjects with allergic asthma, forced expiratory flows (FEF) were recorded on different days, at comparable baseline values, before and up to 5 hr after administration of 8 mg per os (po) chlorpheniramine, 10 mg iv CP (repeated twice), 5.5 mg/kg iv aminophylline, and 30 mg po butabarbital as well as during a day without drug. Chlorpheniramine administered intravenously produced reproducible increases (+ delta) in FEF, starting at 15 min, peaking at 120 min, and still persisting at 5 hr; the peak + delta averaged 15% for FEV1 and 27% to 53% for flows at low lung volume. FEF showed a comparable + delta after aminophylline, a smaller + delta after orally administered chlorpheniramine and no significant + delta during butabarbital or control sessions. The ratio change over time/variability was higher for FEV1, FEF50%, and FEF25%-75% than for the remaining parameters. In six subjects a double-blind study (chlorpheniramine vs. saline solution) confirmed the effectiveness of the doses administered in the open study. In three subjects, 10 mg iv chlorpheniramine was given at four different baseline values; the highest + delta occurred when the basal FEV1 was approximately 50% of the predicted value and the basal FEF at low lung volume 30% to 40% of the predicted value. In two subjects, log dose-response curves to 2.5, 5.0, and 10.0 mg iv chlorpheniramine were obtained by using FEV1, FEF50%, and FEF25%-75%. Thus chlorpheniramine in high iv doses can dilate the bronchi, the + delta FEF depending on the dose, the percent of the predicted basal FEF value, and "individual" responsiveness. Withing the dose range used, bronchodilatation to chlorpheniramine and aminophylline administered intravenously was best detected by FEV1, FEF50%, and FEF25%-75%.