Two methods for the measurement of phosphor-diesterase activity are described, one of which is a method which depends on the selective precipitation of 5’-AMP and the other is a radiochemical method which depends on the liberation of 32Pi from 32P-labeled cyclic 3’,5’-AMP. Rat epididymal adipose tissue appears to contain 10 times higher phosphodiesterase activity than the isolated fat cells derived from the same tissue. Two apparent K values for substrate (2 × 10–6 M and 7.1 × 10–4 m) were found in fat cell homogenate, suggesting the existence of two different enzymes. The inhibition of phosphodiesterase by theophylline shows a complex type of inhibition depending on the substrate concentration.