Otoacoustic emissions in both ears of a rhesus monkey exhibiting stable spontaneous emissions (SOEs) were monitored over a 1-year period. The amplitudes and frequencies of both SOEs and stimulus-frequency emissions (SFEs) were routinely recorded, while transiently evoked (EOE) and distortion-product emissions (DPEs), at the frequency 2f1-f2, were occasionally examined. Between evaluation sessions, both the frequencies and amplitudes of SFEs remained relatively stable in both ears, while the frequencies and amplitudes of SOEs were less constant. Isosuppression contours for SOEs, plotted as a function of frequency and level of tonal maskers, revealed sharp tuning consistent with normal frequency selectivity. Detailed analyses of long-term measurements showed that SOEs occurred most frequently at the peaks of the SFE response. A regular frequency spacing between neighboring amplitude maxima and minima of the SFEs was consistent with the notion that this particular emitted response may result from a periodic disruption of the orderly pattern of sensory cells along the organ of Corti. Intramuscular administration of aspirin abolished SOE and SFE responses, while DPEs remained relatively unchanged suggesting the involvement of separate mechanisms in the generation of different emissions.