A new sensitive method for the quantitative determination of imipramine and desipramine in single rat thyroids using gas chromatography-mass spectrometry with selected ion monitoring, after enzymatic hydrolysis and liquid-liquid extraction has been developed. The technique was deemed suitable for microanalysis of single rat thyroids and for other solid tissues, using smaller sample sizes than usually required for traditional determination methods. The quantification was linear from 10 to 200 nmol/l (i.e., from 0.25 to 5 microg/g) for imipramine and from 100 nmol/l to 2000 nmol/l (i.e., from 2.4 to 47 microg/g) for desipramine, and the limits of detection (less than 25 ng/g tissue for both compounds) were better than those previously reported. Recoveries, repeatability and reproducibility of this technique were satisfactory. It has been successfully applied in a preliminary study of the concentration-time profiles of imipramine and desipramine in the thyroid of rats treated with either of these drugs.