Rat pineal cyclic AMP (cAMP) phosphodiesterases (PDEs) were studied during male development and in adult females by means of gel electrophoresis, kinetic analysis, and diurnal activity measurements. In both male and female rats, two molecular forms of PDE coexisted. In neonatal-infantile males (birth to day 20 of age) a third isozyme was detected. The kinetic properties of PDEs were essentially similar in infantile and adult male rats with respect to the Kms; however, a minimal Vmax was observed a 10.00 a.m. in adults, but not in neonatal animals. PDE activity in male rats was maximal between 2.00 a.m. and 4.00 a.m., and also at 4.00 p.m., and was minimal at 10.00 a.m. This pattern appeared to develop during the infantile period of life. The pattern of activity observed in adult female rats was similar to that of adult males, except that it lacked a pronounced afternoon peak. The diurnal PDE activity profile of castrated male adult rats was similar to female animals and testosterone substitution in male castrated rats resulted in a pattern which resembled that of adult male controls. In addition to the above measurements, testosterone serum levels have been determined during all of these experiments.