The brightness of a field (a test field) can be decreased or increased, when it is surrounded by a brighter or darker field (an inducing field). This phenomenon is called 'brightness contrast'. It seems that this phenomenon is the result of a retinal inhibition. With respect to the neural mechanism of retinal inhibition, a series of investigations were conducted by Hartline and his co-operators. The results they obtained seem to offer profitable suggestions for the physiological understanding of brightness contrast. In the present paper, an attempt has been made to summarize the psycho-physical and physiological aspects of brightness contrast and to examine their relationships (between brightness contrast phenomenon and retinal inhibition). It has been shown that the magnitude of the inhibition, measured in terms of decrease in frequency of discharge, depends upon the intensity, area, and configuration of the pattern of illumination on the retina: (1) the greater the intensity on neighboring receptors, the greater the inhibition they exert on the test receptor; (2) the greater the number of neighboring receptors illuminated-that is to say, the larger the area of illumination, the greater the inhibition exerted on the test receptor; (3) illumination of neighboring receptors nearer to the test receptor rssults in greater inhibition than does illumination of more distant receptors. Comparing psycho-physical facts with these physiological ones, if we replace respectively the word 'inhibition' in the above sentences with the word 'brightness contrast', 'receptor' with 'field', and 'frequency of discharge' with 'apparent brightness', such physiological facts would correspond with the facts of brightness contrast.