Administration of the prototypical mu opiate agonist, morphine sulphate, 1-10 mg/kg, produced over three hours a significant dose-dependent increase in the ingestive responses of free-feeding slugs, Limax maximus, although lower doses, 0.10 mg/kg, attenuated feeding. The mixed mu and kappa opiate agonist, ketocyclazocine hydrochloride, in the dose range 1.0-10 mg/kg, also induced significant increases in food consumption. With both of these opiates there was a latency of about 0.5 hr before initiation of feeding. The more specific kappa opioid agonist, U-50,488H, given over the dose range 0.10-1.0 mg/kg, produced a more potent increase in three hour food consumption by Limax, whereas a dose of 10 mg/kg produced a significant increase in ingestive responses for 3-4 hr after a 1-2 hr period of inactivity. The prototypic mu opiate antagonist, naloxone hydrochloride (1.0 mg/kg) blocked the feeding effects of morphine and ketocyclazocine and reduced the effects of U-50,488H. The delta antagonist, ICI 154,129, in a dose of 10 mg/kg, reduced the effects of morphine as well as decreasing food intake of free-feeding slugs. These results indicate that activation of differential opiate receptors in invertebrates has similar effects on feeding behavior as occur in mammals, suggesting early evolutionary development of opioid involvement in the control of feeding.