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RATIONALE: Experienced cannabis users demonstrate tolerance to some of the impairing acute effects of cannabis. OBJECTIVES: The present study investigates whether event-related potentials (ERPs) differ between occasional and heavy cannabis users after acute Delta(9)-tetrahydrocannabinol (THC) administration, as a result of tolerance. METHODS: Twelve occasional and 12 heavy cannabis users participated in a double-blind, placebo-controlled, crossover study. On two separate days, they smoked a joint containing 0 or 500 mug/kg body weight THC. ERPs were measured while subjects performed a divided attention task (DAT) and stop signal task (SST). RESULTS: In the DAT, THC significantly decreased P100 amplitude in occasional but not in heavy cannabis users. P300 amplitude in the DAT was significantly decreased by THC in both groups. The N200 peak in the SST was not affected by treatment in neither of the groups. Performance in the SST was impaired in both groups after THC treatment, whereas performance in the DAT was impaired by THC only in the occasional users group. CONCLUSIONS: The present study confirms that heavy cannabis users develop tolerance to some of the impairing behavioral effects of cannabis. This tolerance was also evident in the underlying ERPs, suggesting that tolerance demonstrated on performance level is not (completely) due to behavioral compensation.