Bupropion, a noradrenaline/dopamine reuptake inhibitor, and venlafaxine, a serotonin/noradrenaline reuptake inhibitor, are both established antidepressants with proven efficacy in randomized controlled clinical trials. The objective of this double-blind, randomized, placebo- and active-controlled, eight-week, flexible-dose study was to evaluate the efficacy and tolerability of the once-daily extended-release formulations of these two antidepressants compared with placebo. Patients with major depressive disorder were randomized to once-daily treatment with bupropion XR 150 mg (n = 204), the extended-release formulation of venlafaxine (venlafaxine XR) 75 mg (n = 198) or placebo (n = 189) during weeks 1 to 4, with the option to double the dose at week 5 if response was inadequate. In this study, bupropion XR did not demonstrate statistically significant evidence of greater improvement from baseline compared with placebo on week 8 Montgomery Asberg Depression Rating scale scores (primary endpoint) or on secondary endpoints including CGI, HAM-A and responder and remitter analyses. Descriptive statistics for venlafaxine XR indicated separation versus placebo on MADRS total scores at week 8 and other intermediate time points, and on other endpoints including CGI, HAM-A and responder and remitter analyses. Both active treatments elicited improvement on the Sheehan Disability Scale and its subscales and were generally well tolerated at the doses studied. Rates of nausea, dry mouth, dizziness, hyperhidrosis, insomnia, constipation, tremor, anorexia and male sexual dysfunction were elevated in the venlafaxine XR group, consistent with its mixed serotonergic/noradrenergic mechanism. Rates of dry mouth, insomnia and hyperhidrosis were elevated in the bupropion XR group, consistent with its catecholaminergic mechanism.