Twenty-one depressed patients participated in a study that assessed the predictability of amitriptyline (AT) dosage based on plasma drug concentrations after a single dose. In 11 patients maintained on a fixed dose regimen, the 18-h single-dose level was confirmed to be predictive of steady-state levels. The dose for the next 10 patients was derived from their 18-h level aiming to attain steady-state levels of 200 ng/ml. The patients achieved a mean steady-state level of 213 ng/ml with 80% attaining therapeutic levels. All the patients improved within 2 weeks. These preliminary results suggest that dose prediction based on a single-dose TCA level is reliable and beneficial.