The effects of amitraz, a formamidine derivative, on motor function were studied in rats. Behavioural and neurochemical studies were performed. Results show that amitraz was able: 1) to decrease locomotion and rearing frequencies of rats and to increase their immobility time in an open-field; 2) to displace to the left the control dose-response curve constructed to apomorphine-induced stereotyped behavior; 3) to potentiate both pentobarbital sleeping time and amphetamine effects on open-field behaviour of rats and 4) to increase not only the whole brain levels of noradrenaline but also the striatal levels of dopamine. In relation to control animals, the pesticide also induced 5) a decrease in the striatal levels of homovanillic acid, 6) pesticide effects on open-field behaviour of rats were not antagonized by yohimbine administration, and 7) metoclopramide administration on amitraz pretreated rats antagonized not only amphetamine effects on open-field behaviour, but also apomorphine-induced stereotypy. These results are discussed in the light of the actions of amitraz on biogenic amines. It is suggested that the pesticide effects on motor function are consequences of the inhibitory effects on MAO activity, most probably through the increases produced on catecholamine levels within the central nervous system.