The effects of inhaled allergen on airway responsiveness and on beta- and alpha-1-adrenergic receptors on lung membrane were investigated in guinea pigs. After measuring the respiratory threshold to histamine (RT-HIS), one group of guinea pigs passively sensitized for ovalbumin was challenged by allergen inhalation (challenged group). Measurement of the RT-HIS 24 h following challenge revealed a significant decrease from 687 micrograms/ml (mean, n = 16) to 407 micrograms/ml (P less than 0.05). In addition the RT-HIS 24 h after challenge was also significantly lower in the challenged group than in controls (n = 9, P less than 0.05). The density of beta-adrenergic receptors on the lung membrane of the challenged group was 594 +/- 32 (mean +/- SE) fmol/mg protein (n = 11) compared with 712 +/- 24 fmol/mg protein (n = 9) in the controls, a statistically significant difference (P less than 0.05). A significant correlation was found between the RT-HIS and density of beta-adrenergic receptors. From these results, we concluded that the exaggerated airway responsiveness 24 h after allergen challenge is in part due to a decrease in the density of beta-adrenergic receptors. There was no difference in the density of alpha-1-adrenergic receptors nor a significant correlation between the RT-HIS and the number of alpha-1-adrenergic receptors in the challenged vs. the control groups.