Introduction Because toluene is such a ubiquitous substance in our environment??comprising a part of auto emissions and used extensively in industry??there is considerable interest and concern about its possible toxic effects. This concern is especially acute with respect to solvent abusers who voluntarily inhale very high concentrations of toluene for its euphoric effects [1@2]. The high vascularity and high lipid levels of the brain@ and lipid solubility of toluene@ focus concern on possible neurotoxic effects of toluene [3@4]. However@ even though acute neuropharmacologic effects of toluene are readily demonstrable [3@4] there is only limited information about the possible neurotoxic sequelae of subchronic exposures@ although cerebellar ataxia has been attributed to severe toluene abuse [5]. The results of a recent study with rats involving a battery of behavioral and electrophysiologic measures [6] suggested that subchronic exposure to toluene is relatively innocuous with respect to most indexes of sensorimotor neurotoxicity (e.g.@ grip strength@ startle@ sensory-evoked potentials)@ although a region of the brainstem in the vicinity of the inferior colliculus appeared dysfunctional as indicated by the brainstem auditory-evoked response (BAER). Additional study of the performance capacities of rats exposed to toluene led to the discovery of impaired high-frequency M ' hearing in those animals [7]. The study reported here was undertaken to characterize further the hearing loss in these rats by measurement of sound intensity functions of BAER parameters. The BAER is recorded from skull electrodes and consists of a series of potential changes volume-conducted to the skull that are generated in the eighth nerve and brainstem auditory structures (cochlear nucleus@ superior olivary complex@ lateral lemniscus@ and inferior colliculus). By signal averaging the BAER can be extracted from background electrical noise@ As the intensity of the stimulus is decreased@ the latencies to the various components of the BAER increase. and their amptitudes decrease. These characteristics of the BAER have made the response useful in clinical audiometry [8|. In addition@ frequency specificity of hearing loss can be assessed when tone pips of different frequencies@ rather than clicks@ arc used to elicit the BAER [9|