Effects of personal control over noise intensity were studied in experimental situations where subjects performed mental arithmetic under noise exposure. Every other subject was offered a choice between noise intensities, and the next subject, serving as his yoked partner, had to submit to the same noise. Mean measures of catecholamine and cortisol excretion, and of heart rate and subjective effort and discomfort showed that subjects were more aroused in the yoked situation. There were, however, considerable interindividual differences, subjects classified as 'internals' or 'externals' on the basis of the Internal-External Locus of Control Scale responding to the two experimental conditions in congruence with their general beliefs and attitudes with regard to control.