The role of endogenous enkephalins in behavioural control in mice was investigated by i.v. injection of RB 101 (N-[(R,S)-2-benzyl-3[(S)(2-amino-4- methylthio)butyl dithio]-1-oxopropyl]-L-phenylalanine benzyl ester). RB 101 is a recently reported systemically active mixed inhibitor prodrug of the two enzymes which metabolize the enkephalins neutral endopeptidase 24.11 and aminopeptidase N. RB 101 (2.5-10 mg/kg) induced a dose-dependent long-lasting hyperlocomotion and attenuated the conditioned suppression of motility in mice placed in an environment where they had received footshocks 24 h before. In addition, RB 101 decreased the duration of immobility in the forced swim test. All these actions of RB 101 were antagonized by the selective delta antagonist, naltrindole, supporting the preferential involvement of delta opioid receptors in these enkephalin-controlled behavioural responses. The effects induced by RB 101 were also suppressed by prior administration of the selective dopamine D1 antagonist, SCH 23390, but not by the D2 antagonist, sulpiride. Attenuation of the conditioned suppression of motility was associated with increased striatal dihydroxyphenylacetic acid (DOPAC)/dopamine (DA) and homovanillic acid (HVA)/DA ratios, both effects being antagonized by naltrindole. This latter compound is also efficient to inhibit the effect of imipramine in the mouse forced swim test. Taken together, these results support the occurrence of tonic and phasic controls of mood-related behaviour by endogenous enkephalins through delta and D1 receptor stimulation and suggest a possible future use of these mixed inhibitors as new antidepressants.