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Investigations have demonstrated that circadian rhythm can affect animal curiosity, also a significant correlation has been reported between curiosity and addiction. So the aim of this study is to evaluate the effect of circadian fluctuation in curiosity on addiction. For this purpose, we used the open field (OF) test and conditioned place preference (CPP) biased paradigm. In the OF we evaluated animals curiosity in the morning (ZT 1-2) and in the evening (ZT 11-12). In the CPP, during six training sessions held either in the morning or evening, morphine was injected in a specific compartment. Rats were subsequently tested for preferring the drug-paired chamber at the circadian time that either matched or was different from that of training. The evening rats, compared with the morning groups more spent their time in the center of the OF and also showed a higher preference for staying in the white box in CPP apparatus, and if received morphine at this time, they were conditioned more than morning. The finding of this study showed that the curiosity in rats is higher in the evening and zeitgeber time is more potent for conditioned to morphine than in the morning. [ABSTRACT FROM AUTHOR]