The fact that, for a given perceptual-motor performance, a directional transformation of the visual stimuli which feed back and control it, may cause some difficulty to it as in the case of mirror-drawing, suggests a method to measure the directional relationship between visual and tactil-kinesthetic spaces by the degree of difficulty of the performances under such transformations. The method postulates that, among the performances, the one the least transformation should show the least difficulty. Since the results of Exp. I were fairly well interpreted by it, it was called “the law of the least transformation.”The apparatus consistes of a switch matrix and a lamp matrix similar to that used by Boiko and resembles those used in the study of control-display relationship. A response on the switch matrix automatically caused the lighting of a lamp in prearranged order, i.e., Ss turned off agiven lamp by responding to the switch corresponding to it, which resulted in the lighting of another lamp, which the Ss turned off by the appropriate response which in turn caused the lighting of another stimulus and so on.The Ss' task consisted of two successive parts: (1) pracitce sessions during which Ss were shown the switch- and lamp-matrix correspondence (a switching response turned on the corresponding lamp). (2) the pursuitory movement of the hand according to the known switch-lamp correspondence-(in this time, a switching response changed the circuit from the corresponding lamp to another lamp).The combination of the three operations-rotations of the lamp-matrix which varied the direction corresponding to the hand-direction in the visual field (2), arm-positions relative to the body (2), face-directions (2), -gave 8 conditions in Exp. I.The pursuit time of a unit course, randomly chosen from 16P8 and counterbalanced by Greco-Latin square, was adopted as the measure. The variances within conditions were heterogeneous even after the whole data had been transformed into logarithm. The non-parametric median test indicated that the differences between conditions and between individuals (16 university students) were both significant (at 1% level); and as to the differences between the operations, only that between rotations of the lamp-matrix was significant. by T test: arm-positions and face-directions made no significant difference; the directional relationship manifesting itself in the interaction between these operations, should be the main source of the difference between the conditions.It was noted that, by adopting the concept of translation or rotation in the plane such as Fig. 4, the description of the results, became much simpler than by the direct use of the experimental operations.Exp. II, followed Exp. I to show that the hand-direction (i.e., the switch-matrix-direction), which was kept parallel to that, of the body throughout Exp. I, could work as a factor under the law of the least transformation.The nature of the mediating process measured by this experiment, and its function in the pursuitory movement as “an action acceptor” of Anokhin's theory, were briefly discussed.