Cortical oscillations have been reported in relation to animal behavior and may have important roles in neuronal information processing such as "binding". We considered oscillatory activities in the primary somatosensory cortex of rats as a preparatory state for sensory processing and investigated the temporal relationship between gamma oscillation (25-45 Hz) onset in the somatosensory cortex (the barrel cortex) and rat's whisking as tactile exploratory behavior. The gamma oscillations of local field potentials preceded the whisking in 52/83 cases (63%). The mean lead time of the oscillations, defined as time from the oscillation onset to the whisking, was 268 ms (+/- 123, S.D.). These preceding gamma oscillations lasted to the onset of whisking. Furthermore, we investigated the temporal relationship during the other two behavioral states: a still but alert state and a state with tactile stimulation of the whiskers. In the state with tactile stimulation, gamma oscillations followed the stimulation, and in the still but alert state, significant gamma oscillation was not observed. The gamma oscillations that precede the whisking are thought to reflect anticipatory activities in the barrel cortex for the subsequent somatosensory input from the whiskers, which could represent a mechanism for rapid processing of expected sensory information.