Mating behavior is often strikingly dimorphic between the sexes. To gain better insight into the molecular regulation of a motivated sexual behavior, we studied C. elegans mate searching behavior. In the absence of a mating partner C. elegans exhibits a sexually dimorphic behavior. Single males, but not hermaphrodites, leave food at a constant rate and search the surrounding area. This behavior, which is called leaving behavior, is thought of as a mate searching behavior because only animals that need to find a mate to reproduce express it, and when a mate is present, leaving behavior stops. Male mate searching behavior appears to be modulated by a steroid regulated by the gonad; <font face=symbol>D</font>4-dafachronic acid (DA). We have shown that the receptor for DA, DAF-12, is necessary for the gonad to activate mate searching behavior. Since the ligand independent form of daf-12 (m20) activates mate searching in the absence of the gonad, DAF-12 acts at least partially downstream of the gonad signal. A temperature shift experiment showed that DAF-12 functions in adults to regulate mate searching. We wondered if adult administration of the DA precursor (4-cholesten 3-one, which is more readily available than DA itself) could rescue mate searching behavior of gonad ablated animals. Adult administration of the DA precursor could rescue mate searching in animals with the somatic gonad intact (germ line ablated) but not with the somatic gonad removed (gonad ablated). This suggests that the germ line is necessary to provide the DA precursor and the somatic gonad is necessary for the transformation of the DA precursor to DA. DA is necessary during both development and adulthood to stimulate mate searching behavior. DAF-9 produces DA, and daf-9 males share mate searching defects with gonad ablated animals. However, daf-9 males have developmental defects, such as constitutive dauer formation, that are absent in gonad ablated males. This suggests that, with respect to behavior as well, DA may play a gonad independent role during development. Consistent with this idea, daf-9 males reared without DA are insensitive to adult DA supplementation and will not display increased levels of mate searching. However, daf-9 males reared with DA show normal mate searching behavior when given DA as adults (DA was the generous gift of D. Mangelsdorf). This is consistent with the role of sex steroids in other species, which organize sex specific behaviors during development, and activate them during adulthood. In summary, with respect to mate searching behavior DA appears to function as a C. elegans gonadal sex steroid. In adult males DA is either produced or regulated by the gonad, and it both organizes and activates adult male behavior.