Neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) an ancient neurotransmitter, involved in several neurophysiological and behavioral functions, acts by interacting with multiple receptors (5-HT(1)-5-HT(7)). Alterations in serotonergic signalling have also been implicated in various psychiatric disorders. The availability of the genome data of non-human primates permits comparative analysis of human 5-HT receptors with sequences of non-human primates to understand evolutionary divergence. We compared and analyzed serotonergic receptor sequences from human and non-human primates. Phylogenetic analysis by Maximum Likelihood (ML) method classified human and primate 5-HT receptors into six unique clusters. There was considerable conservation of 5-HT receptor sequences between human and non-human primates; however, a greater diversity at the sub-group level was observed. Compared to the other subgroups, larger multiplicity and expansion was seen within the 5-HT(4) receptor subtype in both human and non-human primates. Analysis of non-synonymous and synonymous substitution ratios (Ka/Ks ratio) using the Nei-Gojobori method suggests that 5-HT receptor sequences have undergone negative (purifying) selection over the course of evolution in human, chimpanzee and rhesus monkey. Abnormal human and non-human primate psychopathalogy and behavior, in the context of these variations is discussed. Analysis of these 5-HT receptors in other species will help understand the molecular evolution of 5-HT receptors, and its possible influence on complex behaviors, and psychiatric disorders.