The chronic effects of mianserin and two of its major metabolites on the behaviour of olfactory bulbectomized rats in the ‘open field’ apparatus were studied. When these compounds were tested at the same dose, only the parent compound was active in attenuating the hypermotility which the bulbectomized rat exhibited under these experimental conditions. Changes in the concentrations of noradrenaline and 3-methoxy-4-hydroxyphenylglycol (MHPG) were also determined in the amygdaloid cortex and mid-brain regions of the sham-operated and lesioned rats. Only mianserin reversed the reduction in the noradrenaline concentration which occurred in the amygdaloid cortex following bulbectomy; the changes in noradrenaline or MHPG in the mid-brain were not correlated with this behavioural effect of mianse