The authors have tested the effectiveness of ACTH 1-17, administered in a single dose of 100 micrograms intramuscularly at 07(00) for 10 consecutive days, in 10 male patients suffering from chronic obstructive bronchitis with different ventilatory impairment. The subjects performed every 4 h (02(00), 06(00), 10(00), 14(00), 18(00), 22(00] for 14 consecutive days (3 days of wash-out and 10 days of treatment) spirometric tests with recording of 8 ventilatory parameters and of oral temperature. The day before and after treatment plasma cortisol levels were also determined 6 times a day. After the period of treatment 4 subjects improved, 4 remained stationary and 2 worsened as concerns the ventilatory functions, while 7 perceived a better general condition. The normal circadian synchronization of the respiratory function of the group, with acrophase in the afternoon, was maintained and the temperature rhythm was not influenced. The circadian rhythm of plasma cortisol showed a significant mesor reduction after treatment with a slight advance in acrophase indicating a partial inhibition of the hypothalamo-hypophyseal axis. The success of treatment was independent of the severity of the ventilatory impairment and the subjects showed different patterns of response both quantitatively (probably related to the dosage) and temporally (immediate or delayed). Hyperpigmentation of the skin owing to the intrinsic melanotropic activity of the peptide occurred in 2 subjects. As a side effect, weight increase was apparent in 3 subjects.