||In rats with a unilateral 6-hydroxydopamine lesion of the ascending dopamine neurons, we investigated the relationship between the expression of Fos-like immunoreactivity in the caudate-putamen and contralateral turning behavior in response to dopamine agonists during the induction and expression of sensitization (priming) to D-1-dependent turning behavior. Priming was induced by apomorphine (0.1 mg/kg s.c.) or by SKF 38393 (10 mg/kg s.c.) 14 days after 6-hydroxydopamine lesions and was expressed by challenge with SKF 38393 (3 mg/kg s.c.). In the induction phase of priming, administration of MK 801 (0.1 mg/kg s.c.) potentiated contralateral turning but differentially influenced stimulation of Fos expression in the caudate-putamen by apomorphine and by SKF 38393. Thus, MK 801 reduced in the expression phase of priming the stimulation of Fos expression by apomorphine in the dorsolateral caudate-putamen, but did not affect that by SKF 38393. MK 801, while preventing priming of SKF 38393-induced turning by apomorphine, failed to affect priming by SKF 38393. MK 801, given with apomorphine in the induction phase, reduced the stimulation of Fos expression in the dorsolateral caudate-putamen by SKF 38393. No such inhibitory effect of MK 801 on SKF 38393-stimulated Fos expression was observed in rats primed with SKF 38393. These results are consistent with the possibility that MK 801 disrupts sensitization of D-1 transduction by reducing the activation of c-fos by the DA agonist during the induction phase of priming.