Abstract 1. 1. Computerized analysis of the crabs locomotor behavior revealed an initial increase in activity followed by a gradual decrease over a 12 min observation period. 2. 2. Dopamine, in a dose-dependent manner, inhibits locomotor activity. The effect can be antagonized with the dopamine antagonist, haloperidol. This suggests that dopaminergic influences are involved with locomotor mechanisms. 3. 3. FK 33 824, a stable opioid analog, significantly enhances the initial excitatory locomotor activity. Naloxone, a potent opiate antagonist, can block the excitatory action induced by FK 33 824. This suggests the presence of an opioid modulation mechanism in the regulation of locomotor activity. 4. 4. Concomitant administration of the various agents results in the behavioral characteristics of the agonist appearing when the appropriate antagonist is not present. Thus, administration of dopamine + FK 33 824 + haloperidol results in enhanced locomotor activity. 5. 5. Concomitant dopamine and FK 33 824 administration results in enhanced locomotor activity. This suggests that the opioid mechanism is closer to the last step in affecting the organism's locomotion or in initiating activity.