Tbe authors evaluated the cognitive and psychomotor effects of serotonin reuptake inhibitors in health elderly volunteers. Paroxetine, sertraline, and placebo were compared for 3 weeks of testing in a double-blind study with behavioral testing at baseline and at the end of each week. MANOVA models demonstrated no between-group differences; however, mixed-model random regression analyses revealed that Day 14 plasma paroxetine levels correlated negatively with delayed verbal recall and paired-associate learning scores. In contrast, plasma sertraline levels correlated positively with Day 7 immediate verbal recall, Day 14 tapping, and Day 21 delayed verbal recall scores, and negatively with divided-attention task scores on Day 21. Plasma paroxetine levels were associated with mild behavioral impairment at Day 14, with no other significant adverse effects. Plasma sertraline levels were associated with mild and transient behavioral changes, as well as early termination in several subjects.