||The D-1 receptor agonist, SKF 38393 (2 mg/kg s.c.), failed to elicit contralateral turning when administered to drug-naive rats 17 days after unilateral 6-hydroxydopamine (6-OHDA) lesioning of the medial forebrain bundle, while it elicited intense contralateral turning 90 days post-lesioning. On the other hand the D-1/D-2 receptor agonist, apomorphine (0.1 mg/kg s.c.), induced contralateral turning in drug-naive rats lesioned 14 days earlier and made the administration of SKF 38393 (2 mg/kg s.c.) 3 days later effective to evoke contralateral turning ('priming'). The effectiveness of apomorphine as a primer of SKF 38393-induced turning depended critically on the interval between the administration of the two agonists. Effectiveness was minimal after 3 h and increased after 6-12 h, peaked at 72 h and was reduced after 10 days. The D-2 receptor agonist, LY 171555 (0.2 mg/kg s.c.), was also effective as a primer of SKF 38393-induced contralateral turning and this effect also was dependent upon the interval between priming and SKF 38393 administration. Moreover, priming was dependent on the dose of drug used as primer and on the dose of SKF 38393 used as a challenge. In contrast to SKF 38393, priming was unable to make effective a dose of LY 171555 that was ineffective in drug-naive rats, suggesting that LY 171555 affects D-1-dependent turning to a greater extent than D-2-dependent turning. The results indicate that the priming phenomenon is rather strictly time- and dose-dependent.