We studied the early response to ovalbumin challenge in sensitized Brown-Norway rats through its effect on N₂, He, and SF₆ phase III slopes of the single-breath washout and on indexes of lung function. Sensitized rats showed varying degrees of response in terms of pulmonary pressure (Pl), with increases ranging between 125 and 225% of baseline. The sensitized rats presented decreased quasistatic compliance, forced vital capacity, and end-expiratory flow, with all three lung function indexes showing a significant negative correlation with corresponding Pl values. They also showed significant positive correlations of Pl with the N₂, He, and SF₆ phase III slopes, reflecting diffusion-convection-dependent inhomogeneities generated by conformation changes throughout the entire rat lung. In addition, the rats showing the most marked Pl increases (>150% baseline Pl) also revealed a reversal of the SF₆-He slope difference because of a more marked SF₆ than He slope increase. This latter finding suggests that the degree of structural heterogeneity during early response is even more marked in the most peripheral rat lung generations.