The purpose of this study is to examine the enhancement model of smoking maintenance, which has been proposed by Mangan & Golding (1978). We studied the relationships of the ultradian variations among the smoking behavior and sleepiness. The results were as follows. (1) The results of the time series analysis revealed that several spectral peaks were obtained in day time fluctuations of smoking behavior, sleepiness, mood and task performance. The average peak frequency for each parameter distributed in the range of 10 to 14 cycle/day (c/d : See Fig. 3-6). (2) The significant correlations were obtained between the parameters of the smoking behavior (frequency of puffing and VAS score of need for smoking) and the daytime sleepiness (VAS score). The significant average correlation coefficients between smoking need and sleepiness were observed (See Table 2). In general, present results agree with the enhancement model of smoking maintenance.