We have observed nine asthmatic patients with less than 10% decrease in 1s forced expiratory volume (FEV1) after inhaling acetylcholine. Four of these patients with "intrinsic" asthma were studied before, during and after a spontaneously occurring asthmatic episode. FEV1 was significantly less (P less than 0.02) during airway obstruction than before and after recovery. The average decrease in FEV1 after inhalation of acetylcholine, before and after the obstructive episode, was 5%. In one patient, acetylcholine inhaled during the obstructive episode was followed by only 1% changes in FEV1. Histamine aerosols, administered either before or after the obstructive episode, induced an average drop in FEV1 of 4.7%. A fifth patient with "extrinsic asthma" had a 27% decrease in vital capacity following acetylcholine, during the exposure period to natural allergens, but less than 6% drop in FEV1 outside season when asymptomatic. The average diminution in FEV1 after histamine and acetylcholine was less than 3% in 13 healthy subjects, while in 14 consecutive asthmatics it averaged 40%. We conclude that airway hyperreactivity is not a constant feature in bronchial asthma.