The present study investigated whether the sensitization to acoustic stimuli in pigeons oscillates as a function of the time of day. Both the training and testing were conducted in a single session and the procedure was repeated every 4 h with different experimental and control groups of pigeons. A skeleton photoperiod (two 15-min light pulses to indicate the beginning and the end of the day) was used to prevent the masking effects of the light-dark cycle. The procedure consisted of a baseline exposure to five tone pulses, sensitization training with two electric shocks (5 min interval) starting 5 min later and testing using a series of 40 tone pulses, starting 10 min later. The control birds were exposed to the chamber only. The magnitude of response was measured by an accelerometer located under the floor of the experimental chamber. Sensitization was analyzed by comparing the amplitude of the response during testing to the baseline level. The amplitude of response of the experimental birds varied as a function of the time of day, with significantly greater amplitude during the subjective night (P 5 0.05). Control data showed no daily variation, however, suggesting that the rhythm in sensitization was due to the circadian modulation of a learning process.