Psychopharmacology (2007) 193:85–96 to large reinforcer (delayed reward paradigm), and stop- signal delay (stop-signal paradigm) as within subjects variables using the Statistical Package for the Social Sciences version 11 (SPSS, Chicago, IL, USA). The homogeneity of variance across groups was deter- mined using Mauchly’s tests for equal variances, and in case of violation of homogeneity, corrected, and therefore, more conservative Huynh–Feldt probability values were used for subsequent analyses. In the stop-signal paradigm, some further exploratory analyses were performed between mean go reaction times and the limited hold period using a bivariate Pearson correlation. In addition, the estimated SSRT data were also subjected to a median split analysis to assess whether SR141716A or WIN55,212-2 had differential effects on response inhibition in individuals with relatively “fast” or “slow” stopping abilities. In case of statistically significant main effects, further post hoc comparisons were conducted using Student–Newman–Keuls Tests. The level of probability for statistically significant effects was set at 0.05. 89 Results Effects of SR141716A and WIN55,212-2 on measures of inhibitory response control in the five-choice serial reaction time task The number of premature responses, a measure of inhibi- tory control reflecting impulsive behavior, was dose dependently decreased by SR141716A [Fig. 1a; F3,45 = 12.24, p < 0.001], and further post hoc analyses revealed that all doses significantly lowered premature responding compared with vehicle, whereas the highest dose (3.0 mg/kg) even further lowered the number of premature responses compared with 1.0 mg/kg SR141716A. In contrast, persev- erative responding after correct choice, a different measure of inhibitory control reflecting compulsive behavior, was not affected at any dose [Fig. 1b; F3,45= 1.84, p = 0.15]. Atten- tional function was improved by SR141716A, and further analyses revealed that only 0.3 mg/kg SR141716A signifi- Fig. 1 Effects of SR141716A (a, b) and WIN55,212-2 (c, d) on different measures of inhibi- tory control in the 5-CSRTT. Data depict mean (±SEM) numbers of premature responses (a, c) and perseverative responses after correct choice (b, d). *p <0.05 and **p<0.005 vs vehicle