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Effects of pretreatment with mCPP (A) or Ro 60-0175 (B) on responding maintained by a second-order schedule of stimulus termination in squirrel monkeys (n = 4). Data (mean ± S.E.M.) are expressed as a percentage of responding after administration of the drug vehicle. **, significant difference (p < 0.01) for a given data point compared with vehicle pretreatment. Abscissae, dose of pretreatment. Ordinates, normalized response rate.
