Behavioral-physiological investigations on the sense of taste in primates were carried out by means of two-bottle preference tests. In this pilot study for the first time ever the threshold concentrations of tryptophan were determined in nonhuman primates. The three forms of this amino acid produced only slight differences in performance, except for the remarkebly low sensitivity of Saimiri sciureus to L-tryptophan. The seven nonhuman primate species tested so far avoided L-tryptophan. For D-tryptophan the animals did not show a homogeneous behaviour. The two species of the South American Callitrichidae tested with the racemate preferred it.