Calculations of the response of F region plasma to rapid subauroral ion drifts (SAID) show that the maximum field-aligned drift is a strong function of the time scale over which the SAID is applied. For SAID velocities of 2 km s-1 the peak field-aligned drifts can vary by almost a factor of 10 for differences in the onset time scale from a few seconds to a few minutes. Reasonable considerations of the convective configuration in a SAID event suggest that acceleration times between 10 and 20 min might typically be expected. In such cases, much smaller field-aligned velocities should be observed than would be predicted assuming an essentially instantaneous turn on of the SAID event. ¿ American Geophysical Union 1993