The effect of 45 min systemic heating at 41 degrees C on plasma and RIF-1 tumour pharmacokinetics of intraperitoneally administered melphalan (MEL) was studied in C3H mice. This heat dose causes greater potentiation of MEL in tumour than in marrow cells, resulting in a therapeutic gain for the combined therapy (Honess & Bleehen, 1985). MEL (7.5 mg kg-1) was administered at the start of heating and concentrations assayed from 20-90 min by high-performance liquid chromatography (HPLC). With or without heat peak concentrations were achieved by 20 min and were 3 to 4 micrograms ml-1 in plasma and 1-3 micrograms g-1 in tumour. Higher MEL concentrations in both plasma and tumour were found in heated animals at times after 20 min from injection, but the effect was greater in plasma (2.5-4 fold) than in tumour (1.5-2 fold) where differences were not always significant. At 40 min after a dose of 7.5 mg kg-1, plasma and tumour concentrations in heated animals were equivalent to those after 12.5 mg kg-1 and 8.5 mg kg-1, respectively, without heating. Tumour/plasma ratios were usually lower in heated than in unheated animals where they often exceeded 100%. The apparent plasma elimination half-life (t1/2) was 17.5-25 min in unheated and 24-44 min in heated animals. The area under the curve (AUC) was increased by a factor of 1.2-1.5 in heated animals, at least partly due to a decrease in volume of distribution. The heat induced increase in MEL exposure may be involved in the enhanced response to the drug, but does not appear to explain the therapeutic gain compaired to MEL alone.