Pharmacodynamic data correlating beta-adrenoceptor blockade with plasma level of drug were obtained in healthy male volunteers (3). Direct comparison of the inhibition of isoprenaline induced tachycardia was achieved in each volunteer after the administration of single doses of either acebutolol (300 mg), practolol (400 mg) or propranolol (40 mg). These drugs were approximately equipotent at these doses, at the times of maximum beta-adrenoceptor blockade. Pharmacokinetic data were obtained in hypertensive male patients (4) after treatment with 14C-radioactively labelled acebutolol hydrochloride. Both 'total-14C' levels and specific '14C-acebutol' levels were determined in plasma, urine and faeces. It was shown, by calculation from the renal clearance, that the late biological half-life for the decline of 14C-acebutolol in plasma was 9.4 h and 13.2 h, respectively, in two of these patients treated with a single oral dose (200 mg) of 14C-acebutolol hydrochloride. In one of the patients treated by intravenous infusion (20 mg/10 mn), the late biological half-life plasma was calculated to be 7.5 h. Renal clearance of acebutolol was shown to be close to a mean of 83 ml plasma/mn for each of three patients (two oral and one intravenous) in spite of the fact that one of the orally treated patients had an elevated level of urea in his plasma (47 mg/100 ml) indicative of some impairment of kidney function. The recovery of 14C-radioactivity in the urine (29 p. cent) and faeces (64 p. cent) was 93 p.cent of the dose of labelled acebutolol in one of the orally treated patients. The overall proportion of the dose excreted as unchanged 14C-acebutolol was 62 p.cent. The major metabolite was the product formed by shortening of the butyramido-group of acebutolol to form an acetamido-group. This metabolite was also readily excreted in both urine and faeces and was also detected in an extract of the 4 h plasma from an orally treated patient. It was identified by co-chromatography as the acetyl analogue (M & B 16 942) of acebutolol. It would be detected by the colorimetric assay of acebutolol in plasma because the same aromatic amino-compound (M & B 17 127) would be formed during the acid hydrolysis procedure. A small quantity of an other unidentified metabolite was detected in an extract of freeze-dried urine after autoradiography of a two dimensional thin layer silica-gel chromatogram.