1. Determinations of desipramine in the isolated rat vas deferens were carried out in order to study the uptake of the drug and to evaluate how this may correlate with the effect on noradrenaline (NA)-induced contraction.2. Vasa deferentia, in contact with concentrations of desipramine of 1 ng-25 mug/ml for 10 min accumulated the drug about 6-fold in respect to the medium. When the time of contact was prolonged to 4 h, desipramine (200 ng/ml) was concentrated about 100-fold. The uptake was not saturable and it was not affected by the presence of imipramine, cocaine, ouabain, dinitrophenol and iodoacetate.3. No metabolic process is involved although the accumulation of desipramine is temperature dependent. The release of desipramine from the vas deferens is exponential and it is not affected by the presence of plasma in the medium.4. No clear correlation was found between tissue concentration of the drug and the potentiation of noradrenaline responses, probably because of the high non-specific binding of the drug to tissue components which may mask specific binding sites for NA resulting in potentiation. However, the concentration of desipramine seems to correlate with the inhibition of NA effect which occurs at high doses of desipramine.