In two experiments, properties of sustained and transient mechanisms were studied psychophysically. In the first contrast thresholds were measured for 6 sinewave gratings ranging from 0.375 to 12.0c/deg at 10 durations ranging from 18 to 3000msec. Thresholds were measured in the presence and absence of high contrast 20msec gratings which masked the onsets and offsets of the signals. At 1.5 c/deg and above, the unmasked thresholds decreased as power functions of duration in two stages, reaching an asymptotic level near 1000msec. Below l.5c/deg, the unmasked threshold became independent of duration beyond 100 msec. At all frequencies, the masked thresholds decreased as power functions of duration to 1000msec or more, but the curves for 0.375 and 0.75c/deg never reached the unmasked asymptotic level. In the second experiment, spatial frequency bandwidths were obtained for sinewave gratings ranging from 0.375 to 12.0c/deg. by measuring threshold elevation as a function of the spatial frequency of masking gratings. At 3.0, 6.0 and 12.0 c/deg, the bandwidth functions peaked at the signal frequencies and showed medium bandwidth frequency selectivity. Below 1.5 c/deg, the bandwidth functions exhibited broader spatial frequency tuning, were of higher magnitude, and there was a shift in peak masking to frequencies near 1.0-1.5 c/deg which were above the signal frequencies. The results of both experiments are discussed in terms of the sustained/transient dichotomy.