As soon as they encounter the scent marks made by the urine of fertile male conspecifics, tree shrews of both sexes cover the scent marks with their own sternal-gland secretion by "chinning." The urine of other species, female conspecifics, or castrated male conspecifics does not elicit this behavior; that is, overmarking with sternal-gland secretion is a reaction to a specific chemical signal. The intensity of the overmarking reaction, i.e., the chinning score in response to the urine sample, decreases as the urine is diluted. The urine of different male conspecifics elicits overmarking of varying intensity; an animal's own urine is in general not overmarked. This behavioral reaction meets all the criteria for a bioassay of potential use in isolating the tree-shrew pheromone from urine.