In eukaryotic cells, the inactivation of the cyclic nucleotide signal depends on a complex array of cyclic nucleotide phosphodiesterases (PDEs). Although it has been established that multiple PDE isoenzymes with distinct catalytic properties and regulations coexist in the same cell, the physiological significance of this remarkable complexity is poorly understood. To examine the role of a PDE in cAMP signaling in vivo, we have inactivated the type 4 cAMPspecific PDE (PDE4D) gene, a mammalian homologue of the Drosophila dunce. This isoenzyme is involved in feedback regulation of cAMP levels. Mice deficient in PDE4D exhibit delayed growth as well as reduced viability and female fertility. The decrease in fertility of the null female is caused by impaired ovulation and diminished sensitivity of the granulosa cells to gonadotropins. These pleiotropic phenotypes demonstrate that PDE4D plays a critical role in cAMP signaling and that the activity of this isoenzyme is required for the regulation of growth and fertility. I n organisms as diverse as bacteria and mammals, changes in the intracellular concentration of the second messenger cAMP control a wide range of cell functions including entry and exit from the cell cycle, differentiation, metabolism, and gene expression. Similar to that shown for the Ca 2ϩ -dependent signaling pathway (1), it has been hypothesized that the intensity and duration of the cAMP signals determine the specificity of cellular responses to different stimuli (2). The pattern of intracellular cAMP accumulation depends on the activity of cyclic nucleotide phosphodiesterases (PDEs). These enzymes are regulated during hormone and neurotransmitter stimulation (3, 4) in concert with other mechanisms that control cAMP synthesis (5) and have profound effects on cAMP levels and cell responses (3,4,6,7).To date, at least 20 PDE genes and almost 50 distinct PDE proteins have been described in mammalian cells (8). These isoenzymes are characterized by different biochemical and kinetic properties, subcellular localization, and mechanisms of regulation. However, the reason a cell requires the expression of such a large array of PDEs with an overlapping function is poorly understood. It has been speculated that each isoenzyme serves slightly different roles and its expression is necessary for the specificity of the cAMP signaling or for signal compartmentalization. Here we have addressed the question of this apparent redundancy of the cAMP-degrading machinery by studying the effect of inactivating a single PDE gene.With some rare exceptions, pharmacological inhibition of PDE activity results in an increased signaling through the cAMP-dependent pathway (9), an effect that mimics the stimulation by hormones and neurotransmitters. Although this idea may hold true for short-term pharmacological manipulations, in vivo long-term cell homeostasis and differentiation is more likely impaired by the disruption of the PDE system. If the expression of distinct PDE isoenzymes is indeed required for proper cAMP signaling,...