1 Venous blood was obtained from patients with far-advanced cancer receiving either diamorphine (diacetylmorphine, heroin) hydrochloride (65 samples) or morphine sulphate (24 samples) regularly by mouth in doses from 2.5 mg to 90 mg every 4 h. 2 Samples were obtained within 30 min of the 09.00 h drug round. 3 Serial samples were also obtained over a 4 h period from three patients receiving diamorphine hydrochloride. 4 Assay of serum 'morphine equivalents' was by radioimmunoassay using an antibody that cross reacts almost equally with diamorphine, 6-0 monoacetylmorphine and morphine. 5 The serum concentration of opiates expressed as 'morphine equivalents' ranged from 11 ng/ml to 1440 ng/ml. 6 A highly significant positive linear correlation exists between the dose administered and the serum concentration (P less than 0.001) with respect to both drugs. 7 There was no difference between the two drugs in relation to the serum concentration achieved per 10 mg of opiate administered. 8 Higher oral doses of both diamorphine and morphine are now being used when indicated rather than, as before, resorting to injections when an oral dose in excess of 40 mg is indicated.