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An experiment was designed to determine whether the respiratory retention of sytrene vapor, as estimated from measurements of end-exhaled air, was the same during periods of both constant and fluctuating exposure. Six human subjects were exposed to styrene inside an experimental chamber. A computer-controlled system was used to generate time-varying air concentrations of styrene over 4–5 h in both multistep sequences of constant exposure (four subjects exposed to 15–99 ppm. of styrene in 100-min steps) and fluctuating patterns representative of occupational exposures (two subjects exposed to mean concentrations of styrene of 50 ppm). In the latter case, lognormally distributed exposures, which fit one of two first-order autoregressive models, were generated at intervals of 2.5 min. It was found that the concentration of styrene in end-exhaled air was reduced by about half if the subject inhaled one to three breaths of clean air prior to sampling. This suggests that significant amounts of styrene were desorbed from the lining of the lungs during the initial exhalation. The retention of styrene vapor during constant exposures was 0.935 and was independent of the level. During each of the two sets of fluctuating exposure the retention of sytrene was also constant and was independent of both the variance and the autocorrelation coefficient. However, the retention of styrene during fluctuating exposure (estimates ranged from 0.957 to 0.973) was significantly higher than that observed during the constant exposures. It is speculated that the difference in retention between the constant and the fluctuating exposure regimens is related to non-steady-state behavior of styrene in the richly perfused tissues, as suggested by Opdam and Smolders (1986) regarding tetrachloroethylene exposure.