The strength of active external rotation and of abduction of the shoulder when the humerus was in the plane of the scapula (30 degrees of horizontal flexion anterior to the coronal plane) was measured isokinetically and isometrically in thirty-nine normal volunteers, who were stratified by age and sex. The angles at which peak torque was produced were similar when tested isokinetically and isometrically; these angles were similar for external rotation (at 60 and 30 degrees of internal rotation) and for abduction (at 30 and 60 degrees of abduction). Isometric peak torque was greater than slow-speed (90 degrees per second) isokinetic peak torque, which in turn was greater than fast-speed (210 degrees per second) isokinetic peak torque. There were highly significant differences in strength, measured isokinetically and isometrically, between younger and older men and between older men and older women. The variability of normal values for torque was similar in each group. Repeat testing demonstrated a high reliability of isokinetic measurements and of isometric measurements at angles within the range of the production of peak torque. Complete testing was performed in four normal volunteers before and after a block of the suprascapular nerve. The supraspinatus and infraspinatus components of the rotator cuff contributed a variable proportion to the total strength of abduction (25 to 50 per cent) and external rotation (50 to 75 per cent) throughout the range of motion. This study demonstrated that both isokinetic and isometric testing in the scapular plane are valid methods for measurement of the strength of external rotation and abduction of the shoulder. The data support standardization of the positions for testing the strength of motions of the shoulder: isometric strength of external rotation should be measured in the scapular plane with the shoulder in 45 degrees of abduction and 45 degrees of internal rotation; isometric strength of abduction, in the scapular plane with the shoulder in 45 degrees of abduction; and isokinetic strength of external rotation and abduction, in the scapular plane at 90 degrees per second.