Sixty-three cells in the superior temporal gyrus of awake squirrel monkeys were tested with 8 species-specific vocalizations plus noise, clicks and tones. Identical series of stimuli were repeatedly presented over 1-5 hour intervals. The responses elicited by both vocalizations and artificial stimuli in primary and secondary cortical neurons often varied over time. In several cases the selectivity of a cell to specific vocalizations appeared to change, i.e., a vocalization which was effective in eliciting a response at one point in the experiment, later became ineffective. In the primary cortex 50% of the cells gave variable responses to one or more of the vocalizations. Twenty percent of the primary cortical cells appeared to change the selectivity of their responses to specific vocalizations. In the secondary cortex 62% of the cells varied in their responses to vocalizations; 42% showing apparent changes in selectivity.