Plasma angiotensin converting enzyme was measured in 23 asthmatic subjects before and after administration of prednisolone, 20 mg daily, for seven days. Plasma specimens from seven patients with asthma, seven with sarcoidosis and 14 normal subjects were also assayed before and after the addition of prednisolone in vitro. A plasma free extract of normal lung was also prepared and assayed before and after prednisolone treatment. Mean angiotensin converting enzyme activity was significantly greater in the asthmatic patients (40.3 nmol min-1 ml plasma-1) than in the control population (35.1 nmol min-1 ml plasma-1), though remaining within the 95% reference range. A fall in plasma angiotensin converting enzyme levels was seen after the addition of prednisolone in asthmatics both in vivo and in vitro. Similar in vitro falls were seen in patients with sarcoidosis but not in controls or in normal lung extract. No changes in angiotensin converting enzyme activity were seen after a single dose of 20 mg prednisolone in normal volunteers. Patients with asthma therefore appear to have higher mean angiotensin converting enzyme activities than the normal population and these fall after the addition of prednisolone whether this is added in vivo or in vitro.