Rats were given powdered diet containing L-DOPA (together with the peripheral decarboxylase inhibitor carbidopa) for a period of 6 months. The estimated daily intake was in the range 20-30 mg/kg. Initially, at 1 week and 1 month, L-DOPA-fed rats exhibited enhanced spontaneous locomotor activity, but this fell to within the control range by 3 and 6 months, although (+)-amphetamine-induced hyperactivity was greater at 6 months in L-DOPA-treated animals than in control rats. Six months after receiving L-DOPA in their diet rats showed enhanced stereotypy scores to a series of dopamine agonists administered acutely including (+)-amphetamine, nomifensine, L-DOPA, apomorphine and piribedil compared with the control animals. In another behaviour test L-DOPA administration reduced the cataleptic potency of both fluphenazine and haloperidol was increased. Biochemically 6 months treatment of rats with L-DOPA was associated with significantly increased plasma concentrations of L-DOPA, enhanced striatal levels of L-DOPA, dopamine and dopamine metabolites, enhanced specific binding (as indicated by increased Bmax values) of [3H] spiroperidol, [3H] ADTN and [3H] 5-HT to striatal membranes, and increased basal and dopamine-stimulated striatal adenylate cyclase activity. The results are discussed in the light of changes of sensitivity of cerebral dopamine receptors, an increase in receptor numbers, and the tolerance to L-DOPA which often develop in the treatment of Parkinson's disease.