DBA2 mice show an erratic spontaneous climbing which is reduced by increasing doses of direct dopamine agonists (apomorphine up to 5 mg/kg, piribedil up to 20 mg/kg). Sustained stereotyped climbing occurs when animals are treated with L-dopa plus benserazide and dexamphetamine. In this strain, which is spontaneously insensitive to apomorphine-induced climbing, this behaviour progressively appeared in a stereotyped manner after repeated administrations of apomorphine (5 mg/kg). The sensitization to apomorphine-induced climbing is long-lasting (more than 15 days). A similar sensitization may be induced by repeated administrations of either piribedil or of the dopamine uptake inhibitor GBR 12783. The semichronic reduction in dopaminergic transmission induced by four administrations of haloperidol (2 mg/kg at 2-day intervals) or by pretreatment with reserpine (3 mg/kg) induced sensitization to apomorphine-induced climbing. These results are discussed in terms of modifications in the sensitivity of two types of dopamine receptors exerting opposite effects on climbing behaviour.