Materials and Methods.-FemaleSprague-Dawley rats, 6 weeks old and weighing about 150 gm, were ovariectomized and maintained in a carefully controlled, low-steroid environment for approxi-mately 3 weeks. On the day of experiment, they were matched by body wt into control and ex-perimental groups. Under light sodium pentobarbital (" Nembutal") anesthesia, they were given estrogen in saline1 or control vehicle by saphenous vein. At precise intervals after beginning the injection, uteriwere rapidly excised and immediately plunged into liquidnitrogen. Three uteri from similarly treated animals constituted a pooled sample which was weighed in the frozen state and then pulverized in a stainless steel percussion mortar under liquid nitrogen. The frozen pellet, weighing approximately 300 mg, was homogenized without prior thawing in 8 volumes of ice-cold 5.6% trichloroacetic acid (TCA), and centrifuged at 0WC for 30 min at 3000 rpm. The supernate was promptly frozen at-860C. Although no apparent alteration of cyclic AMP occurred under these conditions up to 90 days, most analyses were carried out within 2-3 weeks. The Breckenridgemicroanalytical procedure,'2 though technically demanding, offers distinct advantages in the direct analysis of the endogenous cyclic AMP content of those intact tissues to which it can be proved" applicable, ie, high sensitivity, precision, and linearity over a useful range of concentrations. It is patterned after the general principles adapted15 for the fluorometric analysis of reduced coenzymes as metabolic indicators. In the present instance, 12 theseare formed at a rate proportional to adenylate concentration, but the yield may be amplified several thousand-fold as a result of cyclic recurrence of coupled enzymatic steps. 16 Enzymes used in the present investigation were obtained commercially as indicated,'2 with the exception of highly specific 3', 5'-cyclic mononucleotide phosphodiesterase.'6 The latter was gen-