The effects of dopamine D1-like receptor agonists, SKF 82958, MCL 201, and MCL 202 on scheduled controlled behavior. Ordinates, response rates expressed as a percentage of that obtained after vehicle injections. Abscissa, cumulative dose of drugs in mg/kg. Each point represents the average effect determined in three to four monkeys. Results are presented as mean % control (+ S.E.M). Note that the R[+] enantiomer of SKF 83959, MCL 202 significantly decreased response rates, whereas the S[−] enantiomer of SKF 83959, MCL 201 failed to alter rates of responding in squirrel monkeys, and that both these compounds were less effective than SKF 82958. * indicates significantly different from vehicle group (Dunnetts t-test, P < 0.05, on means derived from main effect of dose).