Eighteen patients suffering from cancer were entered into a study of the pharmacokinetics and efficacy of methadone and morphine in pain control. All patients had both clinical and radiological evidence of metastatic spread of their cancer and there were no significant differences in age, weight and sites of the primary cancer between the methadone (n = 9) and morphine (n = 9) groups. Blood opioid concentration, visual analogue pain scores (VAPS) and end-tidal percent carbon dioxide were measured before and after both an intravenous and oral dose of either methadone or morphine. Terminal half-lives (mean +/- S.D.) were 30.4 +/- 16.3 h and 2.7 +/- 1.2 h respectively for methadone and morphine while the clearance values (mean +/- S.D.) were 0.19 +/- 0.13 l/min and 1.16 +/- 0.47 l/min. The long half-life of methadone was associated with prolonged pain relief. However, the large variation in the half-life of methadone necessitated careful adjustment of the dosing interval in individual patients. There were pronounced differences in oral bioavailability between the two opioids: methadone, 79 +/- 11.7%, compared to morphine, 26 +/- 13% (mean +/- S.D.). Of greater clinical significance was the variability in these bioavailability estimates with a coefficient of variation of 15% for methadone compared to 50% for morphine. The combined effects of low and variable oral bioavailability for morphine may result in sub-therapeutic doses being administered as practitioners may be inhibited by the size of the effective oral morphine dose and may be confused by the variability in this dose compared to intramuscular doses. The initial oral dose of morphine varied from 15 mg 4 hourly to 150 mg 3 hourly, while the initial dose for methadone varied from 15 mg on alternate nights to 20 mg twice daily. There was no rapid escalation of daily opioid dose for either methadone or morphine when adequate pain control was provided rapidly at the start of treatment by the technique described in this study.