A single injection of 40 mg prednisolone phosphate was given to 10 patients with chronic bronchial asthma. Changes in pulmonary function were followed over a 30-hour period. Statistically significant changes occurred in the tests employed one hour after the injection of prednisolone. The maximum change for the group as a whole was seen to occur after eight hours. This time course of response is very similar to that obtained in previous studies on similar groups of patients with oral prednisolone where the peak effect occurred nine hours after the drug had been given. Intravenous hydrocortisone produces a much earlier peak effect, at five hours, when it is administered to chronic asthmatic patients.