Rats were trained to respond under a cyclic-ratio schedule of reinforcement composed of an ascending, followed by a descending, sequence of ratio values. Subjects were trained while exposed to 70 dB white noise, then tested while exposed to 70 or 90 dB white noise. Exposure to 90 dB white noise elevated the response function (p < .02). Naloxone was then administered intraperitoneally at 0.3, 1.0, and 3.0 mg/kg under 70 dB and 90 dB white noise. Naloxone administration (1.0 and 3.0 mg/kg) significantly depressed the response function obtained under 90 dB white noise (ps < .01) but did not affect the function obtained under 70 dB white noise. These findings suggest that mild stress increases food intake through a mechanism affecting palatability enhanced by the release of endogenous opioids.