The activities of d-amphetamine sulphate, LSD-25, mescaline hydrochloride, indole, tryptophan and riboflavin have been tested on the regenerative powers of the planarian, Dugesia lugubris. Amphetamine completely inhibits regeneration in the posterior pieces and procrastinates regeneration in the anterior pieces of planarians. LSD procrastinates regeneration in both the pieces. Both the substances also manifest a toxic action. Mescaline, indole, tryptophan and riboflavin do not exert any detectable effect. A simultaneous administration of amphetamine and riboflavin attenuates or suppresses the activity of amphetamine; an alternate administration of amphetamine and riboflavin allowes a discrimination between the inhibiting and the toxic action of amphetamine.