To the Editor: We read with great interest the report of Roulston et al. (August 14 issue) about the gradual increase of converting-enzyme activity in stored serum samples that had been taken from patients during treatment with captopril. Studies performed in our laboratory in patients treated with captopril appear to confirm these results. When we used 2-aminobenzoylglycyl-nitrophenylalanylproline as converting-enzyme substrate (Carmel A, Yaron A. Unpublished data), we found a mean increase of 50 per cent in convertingenzyme activity during 10 days' storage at -70°C. At the same time, samples from untreated patients showed constant values of enzyme activity with good. . . No extract is available for articles shorter than 400 words.