To thoroughly investigate the phenomenon of atypical antipsychotic associated weight gain, a feeding laboratory paradigm was developed. This study is a randomized, double-blind, parallel group trial comparing the tolerability and effects of a two-week exposure to olanzapine, risperidone or placebo on weight, resting energy expenditure (REE), and eating behaviors in 48 healthy human subjects. Subjects were randomized to receive olanzapine, risperidone, or placebo and titrated over four days to 10 mg/d, or 4 mg/d, respectively. The mean dose at endpoint was 8.75 mg/day for the olanzapine group and 2.88 mg/d risperidone group. Weight changes were significantly different between groups at midpoint (F = 5.477, df = 2, 44, P = .0001). The olanzapine group demonstrated a significant increase in weight at midpoint (1.59 + 1.80 kg, P = .002) and endpoint (2.25 + 1.62 kg, P = .0001) compared to placebo and at endpoint compared to risperidone (1.05 + 1.15 kg, P = .015). Resting energy expenditures corrected for fat free mass did not reveal any differences between groups. Olanzapine subjects demonstrated significantly more dry mouth and sedation versus placebo while risperidone subjects experienced significantly more sedation, dry mouth, dizziness stuffy nose and restlessness than placebo and more dizziness and stuffy nose versus olanzapine subjects. Thus, a human feeding lab paradigm utilizing a brief exposure to atypical antipsychotics functions as a method to investigate pharmacologically induced weight gain.