A clear interrelationship between biological rhythms and addiction has emerged from recent preclinical and clinical studies. In particular, the manipulation of the so-called 'clock genes' interferes with the manifestation of drug-related responses. For instance, Period 1 (Per1(Brdm1)) mutant mice do not display behavioural sensitization in response to repeated cocaine administration and do not express cocaine conditioned place preference, in contrast to control littermates. To assess the involvement of the mPer1 gene in a robust model of cocaine reinforcement and relapse-like behaviour, we tested Per1(Brdm1) mutant mice and their littermates for self-administration of several doses (0.06-0.75 mg/kg/infusion) of cocaine, and for reinstatement of an extinguished cocaine-seeking response. Per1(Brdm1) mutant mice did not differ from control littermates in their propensity to self-administer cocaine or to reinstate an extinguished cocaine-seeking behaviour in response to drug-associated cues or cocaine priming. In contrast to our earlier data on Per1(Brdm1) mutant mice in cocaine sensitization and conditioned place preference, this finding does not suggest a relationship between the circadian clock gene mPer1 in cocaine self-administration and reinstatement of cocaine-seeking behaviour. This study adds one further example to the notion that various behavioural tests usually used in addiction research rely on different neurobiological substrates.