During therapeutic use of doxepin, we have often observed unexpectedly low doxepin plasma concentrations in patients on moderate dosages, e.g. 100 to 200mg daily. While non-compliance seemed the most likely explanation, we present the data for 6 patients in whom we considered non-compliance unlikely. The data can be explained by hypothesising that in some patients, there is not a linear dosage-plasma concentration relationship and that on a steady dosage, plasma concentrations are not always maintained. If these phenomena can be more carefully documented they may assume clinical importance; indeed for 2 of the patients studied the falling plasma concentrations on a steady dosage were associated with a recurrence of depression.