The effects of the putative neurotransmitters norepinephrine (NE), gamma-aminobutyric acid (BAGA), and acetylcholine (ACh) were tested on auditory cortex neurons which were activated acoustically by species-specific vocalizations in awake squirrel monkeys. Five-barrel glass electrodes were used to record the activity single neurons in the superior temporal gyrus and to apply NE, GABA, or ACh microiontophoretically. Poststimulus time histograms and raster displays of neuronal responses to the vocalizations were computed before, during, and after iontophoresis. Dose-dependent inhibition of spontaneous and vocalization-evoked discharge rates was seen with NE and GABA. Generally, excitation was observed with ACh. A given dose of NE or GABA reduced spontaneous activity by a greater proportion than it reduced activity evoked by the vocalizations. During excitatory responses, segments with lower discharge rates were reduced proportionately more than segments with higher discharge rates. Usually, response 'pattern' was not altered by iontophoresis of any of the substances. However, in some cases the differential inhibition of slow activity produced by NE or GABA did result in a 'patern' change. The demonstration that small amounts of locally applied NE and GABA substantially alter the specific neuronal activation produced by vocalizations provides additional evidence that these agents may function as neurotransmitters in this neocortical area and offers clues about their functional significance.