Unilateral destruction of dopaminergic cell bodies in the substantia nigra zona compacta (SNC) was performed in rats using either electrocoagulation or chemical lesioning (6-hydroxydopamine, 6-OHDA). Neostriatal dopamine concentration ipsilateral to an electrolytic lesion was 34% of the contralateral side 2-3 weeks after operation; serotonin and noradrenaline brain levels were not altered. In contrast, dopamine and noradrenaline forebrain concentrations ipsilateral to a 6-OHDA lesion were 20 and 31%, respectively, of the contralateral side. After 6-OHDA, dopamine concentrations in the ipsilateral neostriatum were reduced to levels below the sensitivity of the fluorometric assay; cortical, brainstem and neostriatal serotonin levels, on the other hand, were not altered after 6-OHDA. Ipsilateral circling behavior was elicited by d-amphetamine after electrolytic and chemical lesioning. In contrast, the direction of circling produced after apomorphine differed between the two lesion models: contralateral circling behavior was exhibited by 6-OHDA-lesioned rats, whereas ipsilateral circling was produced in animals with electrolytic lesions. Contralateral circling was induced in both lesion-type models by haloperidol or pimozide. S.c. atropine administration induced ipsilateral circling in rats with 6-OHDA lesions, whereas contralateral circling was observed after arecoline. Animals with electrolytic SNC lesions turn ipsilaterally after s.c. administrations of either arecoline or atropine. The data indicate that the electrolytic and 6-OHDA circling behavior models represent two different neuropharmacological states and it is, therefore, suggested that comparisons of data obtained from models using different methods of lesioning be made with caution.