Despite increasing knowledge of the neurochemical bases of the action of the tricyclic drugs, little is known about the sequence of psychological effects which precede recovery in drug-responsive patients. This research was aimed at identifying the specific behavioural effects associated with the therapeutic action of amitriptyline in depression. The design involved measurement (post-hoc) of weekly changes in a severely depressed placebo-resistant group who recovered with drug treatment, compared with a group of similar patients treated for the equivalent four weeks, who showed minimal to no clinical response. The research strategy, in accordance with a dose–response paradigm, was to determine which of the early changes in emotion and behaviour found in treatment responders were systematically associated with plasma concentrations of amitriptyline or its major metabolite. Amitriptyline was found to act within seven days on the components of anxiety and on hostility in the responders, and on sleep disorder in all patients. After 12 to 14 days of treatment these effects increased, with improvements in other significant components distinguishing the responders from the non-responders. At the 12th to 14th treatment days when a steady state concentration of drug in plasma was approached, reductions in anxiety and hostility and in certain somatic components correlated significantly with plasma concentrations of amitriptyline. Implications of the findings for clarifying the specificity of clinical actions of the tricyclic drugs, and for understanding the psychobiological dynamics underlying rapid drug-induced recovery in depression, were explored.