The multifunctional phosphoprotein "dopamine and cAMP-related phosphoprotein, M(r) 32,000" (DARPP-32), which is able to act as an intracellular third messenger, was previously found to be present in human luteinized granulosa cells (GCs) and human ovary. DARPP-32 phosphorylation in GCs was increased by dopamine (DA) acting via a DA-1 receptors (D1-R). In the present study, we examined whether the major endocrine signaling molecule for GCs, LH/human CG (hCG), could also affect DARPP-32 phosphorylation. Immunoprecipitation studies showed that hCG, as well as DA, increased phosphorylation of DARPP-32 at threonine residues within 10 min, indicating that the signal transduction pathways of a hormone and a neurotransmitter involve DARPP-32 in GCs. Phosphorylated DARPP-32 is known to inhibit a cellular phosphatase (PP-1), which was also found to be expressed by GCs. Using RT-PCR and sequence analyses we showed that DARPP-32, PP-1, and D1-R genes were not restricted to cultured luteinized GCs, but were expressed in vivo, in the corpus luteum (CL) of the rhesus monkey throughout its entire life span. Whereas hCG increased steroid production in monkey luteinized GCs and in isolated luteal cells, DA failed to affect basal or hCG-stimulated progesterone production. This indicates that, unlike the LH/hCG receptor, the D1-R is not directly linked to steroid production. Although the precise role of D1-R in the CL remains to be shown, the presence of D1-R, DARPP-32, and its target PP-1 in this endocrine tissue, as well as the phosphorylation of DARPP-32 by a gonadotropin and by DA in luteinized GCs, indicate that the signal transduction pathways of the neurotransmitter DA and the gonadotropin hCG/LH involve DARPP-32. The PP-1 inhibitor DARPP-32 may, thus, be a third messenger used by both DA and hCG/LH to exert common regulatory influences on the cells of the CL.