Repeated exposure to amphetamine results in a long-lasting increase in the ability of the drug to elicit locomotor behavior (Robinson and Becker 1986). This sensitized behavioral response to amphetamine (AMPH) in rats depends on a temporal match between the times that sensitizing injections of AMPH are given and the time of testing. A temporal match between training and testing is also required for expression of conditioned place preference (CPP) in rodents. Whereas the sensitization rhythm is abolished if rats are housed in constant light (which produces circadian arrhythmicity), the CPP rhythm persists in arrhythmic hamsters with complete lesions of the suprachiasmatic nucleus (SCN). We now report that the AMPH sensitization rhythm also persists in hamsters following complete SCN ablation. The loss of a sensitivity rhythm in constant light suggests that a disorganized signal being produced by SCN might influence an AMPH sensitive oscillator, affecting the establishment or maintenance of the extra-SCN rhythm.