Psychopharmacology (2009) 205:489–502 from vehicle to drug (morphine, naloxone) and baseline SSRT under vehicle condition. Results Effects of morphine on impulsive action in the five-choice serial reaction time task As depicted in Fig. 1a, c and Table 1, under stable baseline responding, morphine significantly increased the number of premature responses and decreased the number of persev- erative responses after correct choice [F(4,60) = 4.14, ε = 493 0.46, p = 0.030 and F(4,60) = 4.15, ε = 0.67, p = 0.015, re- spectively]. Further comparisons indicated that 3.0 and 6.0 mg/kg morphine significantly increased the number of premature responses, whereas only 6.0 mg/kg morphine decreased the number of perseverative responses. In addition to increasing impulsive action, the highest dose of morphine also significantly increased the number of omissions [F(4,60) = 14.32, ε = 0.65, p < 0.001] and in- creased correct response latencies [F(4,60) = 4.29, ε = 0.60, p = 0.016]. Attentional performance measured by the per- centage accurate choice was not affected by morphine [F (4,60) = 1.09, p = 0.37 and F(4,60) = 2.17, ε = 0.42, p = 0.10]. Likewise, the latency to collect food reward after correct Fig. 1 Effects of morphine (a, b), naloxone (c, d), and their combination (e, f) on different measures of inhibitory control in the 5-CSRTT. In total, n=16 (a–d) and n=15 (e, f) animals were included in the analyses and data depict mean (±SEM) numbers of premature responses (a, c, e) and perseverative responses after correct choice (b, d, f). *p<0.05 versus vehicle and vehicle–vehicle (0–0) injec- tions and +p<0.05, ++p<0.005 versus vehicle/3.0 mg-kg morphine injections