A moderate dose of quinine sulfate, administered to three young adult males, reduced or eliminated various forms of otoacoustic emission (OAE). The individual differences in response to the drug were substantial, but a number of generalizations did emerge. The time courses of onset and recovery were considerably more rapid than for the parallel effects produced by aspirin. Most spontaneous otoacoustic emissions (SOAEs) were eliminated within 7 h of the first 325-mg dose (about 3 h after the second dose). Most SOAEs showed partial or complete recovery about 24 h after the last dose, although considerable instability often remained. The functions relating the magnitude of a distortion-product OAE (DPOAE) to the sound-pressure level (SPL) of the primary tones producing it were displaced toward higher primary levels and became lower sloped following quinine administration. The magnitudes of SOAEs, DPOAEs, and nonlinear peaks in the click-evoked spectra declined and recovered with grossly similar time courses, but there were some partial dissociations. The ability of a DPOAE to suppress an SOAE lying about 50 Hz below it either increased slightly or remained about constant through the drug episode, even though the magnitudes of both DPOAE and SOAE were changing. On several occasions, increases in SOAE magnitude of as much as 10–20 dB were observed during the first 15–30 min of an SOAE measurement period (an initializing effect). Psychophysical measures revealed hearing losses of as much as 20 dB at some frequencies in some subjects. Several short-lived ‘‘enhancements’’ of OAEs are discussed relative to similar quinine-induced effects reported in an animal model.