Abstract Morphine is believed to inhibit the crab's escape response to a danger stimulus due to central drug action. To test alternative explanations of such an effect in terms of afferent and/or efferent impairment, experiments were conducted using the crab's optokinetic response as indicator. Doses of morphine with maximal detrimental effect on the escape response (75–100 μg/g) showed no effect on the optokinetic response, both by measuring the crab's eyestalk displacement and by recording its body rotation, supporting the hypothesis of a morphine central action on the danger-induced escape re sponse. As regards the effet on habituation, a 75 μ morphine/g injection administered 30 min before the first trial produced a parallel shift of the short-term (within-session) habituation curve, suggesting a modulatory central drug action that would mimic a putative endogeneous opioid action. A 100 morphine μg/g dose injected 30 min before training sharply reduced reactivity during training and impaired the acquisition of long-term (between-session) habituation. It may be speculated that the decrease in the danger meaning of the stimulus due to morphine explains both effects in terms of a stimulation impairment during training.