1 The hypotensive effect of single daily dosing with atenolol 100 mg and chlorthalidone 25 mg given alone or in combination has been assessed in a double-blind, crossover, placebo controlled trial in fifteen hypertensive patients. 2 Average lying blood pressures were: Placebo 155.4/103.9 mm Hg, atenolol 134.6/85.8 mm Hg, chlorthalidone 139.5/90.1 mm Hg, combination 127.7/82.5 mm Hg. 3 The effect of the combination therapy in reducing lying diastolic pressure compared with placebo (a fall of 21.4 mm Hg) was significantly less than the 31.9 mm Hg fall predicted from the sum of the individual effects (P = 0.01). 4 Observations on blood pressure at rest and under mental, isometric and bicycle ergometer stress were made pre-dose and post-dose for a 12 h period at the end of the last treatment period. 5 Lying blood pressure declined from the zero hour (pre-dose) reading on all treatments to a low at 15.00--18.00 h and then rose again. 6 The rise in systolic blood pressure after isometric exercise and mental stress was of a similar magnitude with all four treatment regimes. 7 Atenolol, alone and in combination with chlorthalidone, reduced the blood pressure and the pulse rate increase on exercise 2 h post-dose when compared with readings 24 h post-dose. 8 Once daily dosing with a combination of atenolol and chlorthalidone produced a fall in supine blood pressure over a 24 h period but the effect on exercise induced changes was not uniform over this period.