Haloperidol and risperidone caused catalepsy in both wild-type (WT) (N=19–33) and knockout (KO) mice (N=21–31) in a dose-dependent manner (a). At doses of 0.4 mg/kg haloperidol and risperidone both induced catalepsy relative to vehicle-treated mice. Clozapine 5 mg/kg (N=22 WT, 19 KO), however, did not induce catalepsy in either genotype (b). The serotonin 2A (5-HT2A) antagonist AC90179 (c) (N=21 WT, 23 KO) and the 5-HT2C antagonist SB242084 (d) (N=34 WT, 30 KO) alone did not affect catalepsy nor did they alter the cataleptic response to haloperidol (N=9–13). Mice treated with risperidone and SB242084 (N=11 WT, 10 KO) showed significantly less catalepsy than haloperidol-treated mice (e: N=10 WT, 12 KO). They were also significantly different to mice treated with vehicle (N=10 WT, 9 KO) (e). Co-administration of SB242084 and AC90179 with haloperidol (N=10 WT, 11 KO) also attenuated catalepsy. However, catalepsy levels were still significantly higher than vehicle-treated mice (N=10 WT, 9 KO) (e). Data are shown as mean±SEM. ***P<0.0001 difference from vehicle treatment.