In contrast to hermaphrodites, single C.elegans males when cultured on a small bacterial lawn do not remain on the food source indefinitely. Eventually an isolated male wanders off the food source. We refer to this sexually dimorphic behavior as leaving. We have characterized leaving and found that its properties are consistent with that of a spontaneous, goal-directed and regulated mate searching behavior. Leaving behavior is only expressed by adult males and its rate is governed by internal factors like nutritional status as well as external factors like presence of other males or hermaphrodites. We have developed a simple assay to quantitate this leaving behavior. We found that leaving is a stochastic process definable by a decay constant PL , which is the probability of leaving per hour. Leaving behavior, at least in part, should be dependent on locomotion. So the differences in locomotion of different mutants used in our study may be enough to account for the differences in PL and this might also account for the sexual dimorphism since males are more active than hermaphrodites. To address this issue , we used the leaving assay and demonstrated that there was apparently no correlation between activity (as determined by body bends per 20 seconds) and the probability of leaving in various mutant backgrounds. These observations support the hypothesis that leaving is not a secondary consequence of locomotion but is a specific regulated sexual behavior. The properties of leaving and the availability of a simple quantitative assay prompted us to investigate the genetic basis of this leaving behavior. We carried out a genetic screen for mutants with decreased PL values. So far we have isolated several las (leaving assay defective ) mutants. One of these mutants is an allele of the C.elegans serotonin reuptake transporter (CeSERT) gene, mod-5, which functions in serotonergic neurotransmission to clear the synaptic cleft of released serotonin. This suggests that modulation of serotonin and/or components of the serotonin pathway may play a critical role in leaving behavior. The finding is of particular interest because blocking the serotonin reuptake transporter in humans (with fluoxetine antidepressants) results in decrease in sex drive. It is interesting to note that of the three previously identified mod-5 alleles one of them has a relatively low PL value. We are currently investigating the genetic complexity at this locus that may give rise to such a result.