A series of experiments on human subjects assessed the effects of pitch and roll and habituation on motion sickness incidence (MSI). Pitch and roll angular accelerations, even larger than expected at sea, failed to systematically increase MSI. Habituation was evidenced in susceptible subjects who received consecutive daily 1-hour or 2-hour exposures to vertical motion. Habituation was greater for the longer exposure and the more severe motions. A mathematical model describing MSI as a function of the frequency and acceleration of vertical oscillation was refined by including exposure time as an independent variable. Investigation of frequencies of oscillation above .5 Hz confirmed the prediction of the model that MSI continues to decrease as a function of frequency for all frequencies greater than approximately .16 Hz.