Eight experiments are described that compare pattern adaptation and forward pattern masking by examining the effects of five variables on the contrast threshold of a target presented after an adapter or masker. The target is a Gabor pattern with a center frequency of 2 c/deg and a duration of 33 msec. Thresholds are determined using an adaptive spatial forced-choice method. Principal results are as follows. (1) An adapt-refresh regime with a 2 sec refresh and a 2 sec recovery period on each trial is shown to maintain constant performance. (2) Desensitization is very rapid, reaching near maximum in < 200 msec. (3) Recovery is very rapid during the first 100-200 msec and then very slow with the rate of slow recovery decreasing as adapter/masker duration increases. (4) Threshold vs contrast functions are step-like for certain frequency pairs. (5) Sensitivity vs frequency functions derived from adapting and masking are similar in form. (6) Masker temporal frequency (0-15 Hz) has very little effect. These results are described by a theory that postulates that the target is detected by a few mechanisms that are differentially tuned to spatial frequency. The effect of both a forward masker and an adapter is to desensitize the mechanisms that respond to it. Recovery is a weighted sum of two decay processes, one fast and one slow. The theory fits the data from both paradigms well with some differences in parameters.