Three adult male baboons were trained on a psychophysical procedure to discriminate five synthetic, steady-state vowel sounds [a), (ae), (c), (U), and (epsilon] from one another. A pulsed train of one vowel comprised the reference stimulus during a session. Animals were trained to press a lever and release the lever only when this reference vowel sound changed to one of the comparison vowels. All animals learned the vowel discriminations rapidly and, once learned, performed the discriminations at the 95%-100% correct level. During the initial acquisition of the discriminations, however, percent correct detections were higher for those vowels with greater spectral differences from the reference vowel. For some cases, the detection scores correlated closely with differences between first formant peaks, while for others the detection scores correlated more closely with differences between second formant peaks. Once the discriminations were acquired, no discriminability differences were apparent among the different vowels. Underlying discriminability differences were still present, however, and could be revealed by giving a minor tranquilizer (diazepam) that lowered discrimination performances. These drug-induced decrements in vowel discriminability were also correlated with spectral differences, with lower vowel discriminability scores found for those vowels with smaller spectral differences from the reference vowel.