Two studies were conducted to examine the effects of oral L-tryptophan (TRP) supplementation as a treatment for self-injurious behavior (SIB) and to investigate behavior Meyer-Holzapfel 1968;Jones and Barraclough 1978). In humans, SIB has been described as representing a continuum of behavior ranging from occasional noninjurious biting, scratching, and head banging to persistent self-biting and cutting associated with severe physical injuries (Harris 1995;Walsh and Rosen 1988). SIB also occurs in rhesus monkeys, most often as a result of self-biting (Tinklepaugh 1928;Erwin et al. 1973;Capitanio 1986). A retrospective study documenting the incidence of wounding in three primate colonies (Bayne et al. 1995) showed that, of 253 monkeys studied, 0.8% engaged in self-injurious behavior that resulted in wounds requiring veterinary treatment. Although severe self-biting in nonhuman primates is a relatively uncommon phenomenon, it nevertheless remains a serious concern, because it can lead to severe and sometimes fatal injuries.