Escitalopram is the active S-enantiomer of citalopram. In a chronic mild stress model of depression in rats, treatments with both escitalopram and citalopram were effective; however, a faster time to onset of efficacy compared to vehicle treatment was observed for escitalopram-treated (5 mg/kg/day) than for citalopram-treated (10 mg/kg/day) rats at Week 1. To study the predictability of this observation in the clinic, we analysed 4-week data from an 8-week, double-blind, randomised, placebo-controlled, flexible-dose study that compared escitalopram and citalopram to placebo in primary care patients with major depressive disorder (baseline Montgomery and Asberg Depression Rating Scale (MADRS) scores > or =22 and < or =40). Since the flexible dosing started after Week 4, analysis of 4-week data ensured that the patients received fixed doses of 10 mg/day escitalopram (155 patients), 20 mg/day citalopram (160 patients), or placebo (154 patients). The efficacy analysis showed a significantly superior therapeutic effect for escitalopram versus placebo from Week 1 onwards (observed cases) with an adjusted mean change in MADRS at Week 4 (last observation carried forward) of 2.7 points (P=0.002). By comparison, 20 mg/day citalopram did not demonstrate a statistically significant effect compared to placebo. Escitalopram was well tolerated with an adverse event profile similar to that of citalopram. The preclinical observation that escitalopram possesses a faster time to onset of efficacy than citalopram was also seen in primary care patients with major depressive disorder. Thus, escitalopram is efficacious in depression and the effect occurs earlier than for citalopram.