The activation of 5-HT 2A receptors has been shown to enhance the probability of premature responding, regarded as a form of motor impulsive behaviour. At the behavioural level, the interaction of a-adrenoceptors and 5-HT 2 receptors has been linked to head twitch behaviour, regarded as an experimental model of compulsive behaviour. The aim was to determine whether the probability of premature responding induced by an excess activation of 5-HT 2A receptors can be modulated by the blockade of a 1 -or a 2 -adrenoceptors. In the experiments, the 5-choice serial reaction time task was used to measure attention and response control of the rats. The experiments assessed the effects of (∫)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI) 0.1-0.2 mg/kg subcutaneously, a 5-HT 2A/2C agonist, and prazosin, an a 1 -adrenoceptor antagonist, alone or in combination, on the performance of rats. In an additional experiment to examine the possible role of the a 2 -adrenoceptors, a potent, selective and specific a 2 -adrenoceptor antagonist, atipamezole, was given alone or in combination with DOI. Results showed that DOI increased the probability of premature responses, but it did not affect the choice accuracy. Prazosin (0.1 or 0.3 mg/kg, subcutaneously), given on its own had no effects on probability of responding prematurely, but prazosin (0.3 mg/kg.) was able to attenuate the DOI-induced responding. Atipamezole (0.1 mg/kg, s.c.) did not attenuate the effect of DOI on probability of premature responding. When given at lower doses, DOI (0.03 mg/kg) and atipamezole (0.03 mg/kg) synergistically increased the probability of premature responding, whereas a higher dose of atipamezole (0.3 mg/kg) on its own increased the probability of responding prematurely, but this effect was not additive to that of 0.1 mg/kg DOI. These data indicate that 5-HT 2 receptor activation enhances impulsive responding and this effect can be diminished by the blockade of a 1 -adrenoceptors. Atipamezole, an a 2 -antagonist, enhances the probability of premature responding and shares the mechanism of action with the 5-HT 2 agonist in this respect. These results provide evidence for an interaction between the serotonergic 5-HT 2 receptors and aadrenoceptors in the modulation of response control to the motor impulsivity type of behaviour (premature responding) in addition to that of compulsory behaviour (head shakes) found previously.