A gas chromatographic/mass spectrometric method (using either electron impact or chemical ionisation with methane or ammonia) is described for the quantitative analysis of maprotiline (MP, Ludiomil), N-acetylmaprotiline (AcMP) and N-acetyldesmethylmaprotiline (AcDMP) in whole blood or plasma. In two groups (A and B) of 82 and 53 depressive patients treated clinically with MP for 10 and 21 days, respectively, plasma and whole blood MP was monitored during the treatment. In group A, all subjects were phenotyped with debrisoquine and mephenytoin, and 44 with sulfamidine. 5 patients were poor metabolizers of debrisoquine and 2 of mephenytoin; 18 subjects were fast acetylators of sulfamidine. Traces of AcMP were found only in two patients. AcDMP was present in levels below 2 ng/ml in the plasma of most of the patients in group A. In group B, AcDMP levels between 2.4-14.6 ng/ml of whole blood were found in 9 patients. The mass spectral data suggest the presence of another, unknown MP metabolite interfering partly with the analysis of AcDMP. The presence of AcDMP seemed not to be related to the phenotype of the patients as determined by the pharmacogenetic tests.