The effects of constant and variable stimulus intensity on simple RT were investigated with an effective intensity manipulation. Luminances were 40 and 90 dB re 10(-10) lambert. RT was generally longer when intensity varied within a session than when it was constant. RT also was longer when a bright stimulus was presented on the preceding trial as opposed to a weak one. These effects were not found in earlier studies in which differences in luminance were small. However, the RT-difference between bright and dim stimuli was equal under the conditions of constant and variable intensity. Hence the present results replicate only in part the auditory findings of Grice and Hunter (1964) and Thrane (1961).