Temporal patterns of smoking topography were studied in the natural environment. Six smokers smoked all cigarettes over a 10-day period using a small portable electronic recording device which allowed the measurement of a number of aspects of smoking topography. When the data were averaged in 2-hour blocks across the 10-day smoking period, all subjects except one showed differences in smoking topography as a function of time of day. There was considerable intersubject variability in the aspects of smoking topography which varied over the smoking day, and subjects also differed in the pattern of changes seen.