A small (20 × 11 × 6 cm), battery-powered, fully portable device is described for administering a four-choice serial reaction time test and recording the results on a standard magnetic tape cassette. The mains-powered decoder through which the tape is subsequently replayed is also described. The main application is the self-administration of the test by the subject in the field, particularly in studies of environmental stress, the data-bearing cassettes being passed to the experimenter for laboratory-based analysis. In preliminary performance trials, the four-choice test appeared to reflect fatigue due to continuous repetitive responding in a way similar to classical, nonportable, multiple-choice serial reaction tests, but over a shorter time scale and with greater internal consistency.