Diamine oxidase (EC 1.4.3.6) activity, measured as [14C]delta1 -pyrroline formation from [14C]putrescine, was studied in homogenates of rat kidney during compensatory hypertrophy after unilateral nephrectomy. Acetaldehyde and to a lesser degree phenobarbital, at concentrations which did not modify the activity of a preparation of hog kidney diamine oxidase, increased delta1 -pyrroline formation in kidney homogenate, which suggests that aldehyde-metabolizing enzymes present in this tissue may interfere with the yield of delta1 -pyrroline formation and that the use of acetaldehyde may give better information on kidney diamine oxidase activity. Other inhibitors of aldehyde-metabolizing enzymes such as chloral hydrate, disulfiram, and pyrazole cannot be used for diamine oxidase determination since they stimulated or depressed this enzyme activity. In rat kidney undergoing compensatory hypertrophy the levels of putrescine, spermidine, and spermine increased rapidly and were followed by an increase in diamine oxidase activity that presented a first peak on day 2 and a second peak on day 6. The administration of cycloheximide or actinomycin D to nephrectomized rates prevented the increase in diamine oxidase activity. The study of the turnover rate of diamine oxidase with cycloheximide demonstrated that the half-life of this enzyme was about 14 h in normal and hypertrophic kidney. The results suggest that the increase in diamine oxidase activity in renal hypertrophy was due to the synthesis of new enzymes rather than to slowing of its degradation.