Initial work reporting elevated polyamine levels in body fluids of cancer patients indicated that a percentage of the polyamine pools was present in conjugated form making hydrolysis necessary for assessment of the total polyamine content in urine and serum. In this paper, we report plasma decay curves for [14C]polyamines after i.v. administration and the temporal appearance of conjugates. Following the administration of [14C]polyamines, the radiolabel rapidly disappeared from the plasma in the order; spermidine greater than putrescine greater than spermine. Separation of the [14C]polyamines from conjugated radiolabeled compounds with Dowex chromatography indicated that [14C]putrescine and [14C]spermidine were rapidly conjugated, whereas no significant conjugation of spermine was detectable. After near-total hepatectomy of rats, there was no detectable formation of conjugates, whereas unilateral nephrectomy had little effect on the appearance of conjugates. This suggests that conjugation may take place in the liver. Free putrescine or spermidine could be regenerated from the conjugates by acid hydrolysis, suggesting that the conjugation process does not involve any alteration of the polyamines.