A comparison was made between the performance, on a five‐choice serial reaction time task, of three groups of twenty dysthymic, twenty hysteric, and twenty normal subjects. Substantial confirmation was found for the main hypothesis that dysthymics and hysterics would differ in arousal level, as reflected in their speed of performance on the task. As predicted, dysthymics performed at a significantly faster rate than hysterics, with normals intermediate. The effects of inhibition on subsequent performance were measured by means of fall‐off, reminiscence, and error scores. Since the absolute amount of inhibition built up during the task was partly a function of initial speed, the influence of the latter was taken into consideration in assessing group differences on these three variables. As a result, differences between the groups were found to be in line with expectation, hysterics showing relatively greater fall‐off, reminiscence and errors than dysthymics, normals being intermediate on the first two of these measures. Since differences between the three groups in extraversion were not sufficient to account for these findings, and in view of the apparent arousal differences, the hypothesis is suggested that there is, in dysthymics and hysterics, an effective shift in the excitation‐inhibition balance proposed by Eysenck to underly the personality dimension of introversion‐extraversion. Arousal and inhibition are thus seen as interacting processes, the behaviour of hysterics and dysthymics being extremes of this interaction. This hypothesis is discussed in relation to the recent revival of interest in activation theory, particularly those aspects which regard anxiety, arousal, and drive as having a common neurophysiological basis.

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