Intracerebroventricular administration of P-octopamine (OA) had opposite effects on locomotor activity depending on whether or not the rats were subjected to uncontrollable electric shocks. In unshocked rats, OA produced a large decrease in locomotor activity, but when the rats were subjected to unsignalled and uncontrollable electric shocks, a significant increase in locomotor activity resulted. The latter effect was observed either when the shocks were applied during the measurement of locomotor activity or when they were applied the day before (conditioned suppression paradigm). These results support the hypothesis of a neuromodulation of central noradrenergic transmission by octopamine.