The results of pharmacokinetic studies of two recent 5-HT uptake inhibitors, zimelidine and fluoxetine, have pointed to the inadequacy of open-dose rising studies for establishing the most appropriate dose of new antidepressants. High plasma concentrations of the active metabolites, norzimelidine and norfluoxetine, were associated with a poorer therapeutic response in patients suffering from major depression. High drug plasma concentrations are also associated with increased side effects. Large fixed-dose placebo controlled studies with fluoxetine have confirmed the findings of the small pharmacokinetic study that lower doses are more effective. Fixed-dose pharmacokinetic studies are recommended as part of the program to establish the best dose of new antidepressants.