N-[2-(dimethylamino)ethyl]acridine-4-carboxamide (AC) is an experimental antitumour agent that is being considered for phase I trials. After i.p. administration of 150 mg/kg [3H]-AC to tumour-bearing mice, AC was absorbed rapidly into the plasma and tissues such as the heart, liver, kidney and brain but more slowly into the s.c. tumour. The maximal AC concentration (86 +/- 36 mumol/kg) in the tumour occurred at 35-60 min and was 3-fold the maximal plasma concentration, which occurred at 15 min. Although higher maximal concentrations were observed in other tissues, these concentrations fell rapidly in parallel with plasma concentrations. In contrast, AC concentrations in the tumour remained elevated, the t1/2 value (16.3 h) and mean residence time (MRT, 9.5 h) being prolonged in comparison with those in the plasma and other tissues (t1/2 range, 1.0-2.9 h; MRT, 1.2-1.4 h). AC concentrations were not detectable by our high-performance liquid chromatographic (HPLC) method (limit of detection, 0.02 mumol/l) in the plasma or other tissues at 24 or 48 h after administration but were measurable in the tumour (1.6 +/- 0.8 and 0.6 +/- 0.3 mumol/kg, respectively). Radioactivity concentrations in the plasma, tissues and tumour were very variable but were greater than the corresponding levels of unchanged parent AC. By 24 h, radioactivity concentrations in the plasma, tissues and tumour had fallen to similar levels with prolonged elimination profiles. Thus, the exposure of the s.c. implanted tumour to a threshold AC concentration for a prolonged time (> 24 h) tumour, whereas the shorter period of exposure of blood and other tissues may explain its low haematological toxicity.