The intracellular second messenger cAMP affects cell physiology by directly interacting with effector molecules that include cyclic nucleotide-gated ion channels, cAMP-regulated G protein exchange factors, and cAMP-dependent protein kinases (PKA). Two catalytic subunits, Cα and Cβ, are expressed in the mouse and mediate the effects of PKA. We generated a null mutation in the major catalytic subunit of PKA, Cα, and observed early postnatal lethality in the majority of Cα knockout mice. Surprisingly, a small percentage of Cα knockout mice, although runted, survived to adulthood. This growth retardation was not due to decreased GH production but did correlate with a reduction in IGF-I mRNA in the liver and diminished production of the major urinary proteins in kidney. The survival of Cα knockout mice after birth is dependent on the genetic background as well as environmental factors, but sufficient adult animals were obtained to characterize the mutants. In these animals, compensatory increases in Cβ levels occurred in brain whereas many tissues, including skeletal muscle, heart, and sperm, contained less than 10% of the normal PKA activity. Analysis of sperm in Cα knockout males revealed that spermatogenesis progressed normally but that mature sperm had defective forward motility.