The ear bar of a stereo-taxic instrument was modified to permit the introduction of acoustic stimuli to the rat's eardrum during acute experiments on the auditory nervous system. For measurement of the sound pressure of pure-tone stimuli, a probe microphone was attached to the hollow ear bar so that the probe tube tip is placed near the eardrum. Pure tones are led through a fiber tube in the wall of the experimental chamber to the hollow ear bar. Tones between 100 and 15 000 Hz are produced by an electromagnetic speaker, while higher frequencies are generated by an electrostatic transducer. Acoustic transients are produced by a 1-in. condenser microphone operated as a loudspeaker attached to the hollow ear bar inside the experimental chamber. The acoustic waveform of these transients was measured with a 14-in. condenser microphone situated at the end of a 0.08-cc coupler, which is placed on the hollow ear bar to take the place of the rat's bulla. The three sound-producing devices can be interchanged without disturbing recording electrodes or the probe microphone. [Research was supported by the National Institutes of Health and Middlebury College.]