Rate of fluctuation of figures drawn in equivocal perspective was studied in parallel with curves of out-put, and subjective estimates of strain and effort. Fatigue, in the general sense, is found to increase the rate of fluctuation of such figures, and this finding contrasts with statements in the literature to the effect that fatigue produces slower changes. The results call for a revision of conclusions concerning the relation of this phenomenon also to certain other variables, such as the influence of drugs, practise, and voluntary control.