Effects of aerosol inhalation of prostaglandins F_2α and E₂ on specific airway conductance were studied in 12 healthy subjects and 17 patients with bronchial asthma. Approximately 50 per cent reduction of specific airway conductance was obtained with 250 to 1,000 µg of prostaglandin F_2α in healthy subjects and 4 to 512 ng of prostaglandin F_2α in asthmatics. In healthy subjects, repeated administration of prostaglandin F_2α within 1 session elicited nearly identical changes in specific airway conductance, whereas a degree of tachyphylaxis possibly existed in asthmatics. The sensitivity to prostaglandin F_2α during a 17-month period remained quite constant; test-retest responses did not differ significantly. Pretreatment with 1 mg of atropine did not modify the effect of prostaglandin F_2α in healthy subjects. In contrast, bronchoconstriction in asthmatics was diminished 1 and 5 but not 10 minutes after inhalation of prostaglandin F_2α. Prostaglandin E₂ was a bronchodilator in healthy subjects, but caused either dilation or constriction (the direction of the change in specific airway conductance being consistent for each subject) in asthmatics.