Male mice will emit ultrasounds to at least two chemosignals present in female mouse urine: a) an ephemeral unconditioned stimulus (UCS) present in freshly voided urine and b) a conditioned stimulus (CS) that remains after the UCS has disappeared. In this study, four phases examined the time frame over which the UCS disappeared. Urine that had been aged for either 12, 15, 18, or 24 h was compared for vocalization elicitation with fresh urine (which contains the UCS) and metabolic cage-collected urine ( which does not). Under the conditions utilized, significant pheromone fade-out occurred between 15 and 18 h.