Here we describe, for the first time, the occurrence of behavioural sensitization after chronic exposure to Δ9-tetrahydrocannabinol. Rats were treated twice a day, for five days, with increasing doses (5, 10, 20, 40, 40 mg/kg i.p.) of Δ9-tetrahydrocannabinol or its vehicle and after 20 days of withdrawal, animals were challenged with 5 mg/kg (i.p.) of the drug and their behaviour was assessed. Contrary to the motor inhibition induced in control rats, challenge with Δ9-tetrahydrocannabinol in pre-exposed animals elicited a complex behavioural syndrome mainly characterized by oral stereotyped items. Due to the relevance of behavioural sensitization in drug-seeking behaviour that persists long after discontinuation of drug use, our findings suggest that cannabinoids could trigger neurobiological alteration not dissimilar from those observed with more harmful abused drugs.