Spatial-frequency discrimination thresholds were measured using the temporal two-alternative forced-choice procedure. A superimposed masker grating reduced discrimination threshold more than 20-fold when the spatial frequency of the masker was close to the test spatial frequency because moiré fringes provided a sensitive cue to the test grating's spatial frequency. When the effectiveness of this cue was reduced by randomly changing the frequency of the masker grating on successive presentations, masked thresholds were found to be elevated when the masker frequency was within roughly 1 octave of the test frequency. On the other hand, postadaptation thresholds were not elevated when the test frequency was close to the adapting frequency but were elevated at about twice the adapting frequency. Thus masking and adaptation have quite different effects on spatial-frequency discrimination, though their effects on contrast sensitivity are qualitatively similar. It is proposed that discrimination is determined by opponent-size processing. Adaptation elevates discrimination threshold by reducing the sensitivity of opponent elements. The masker grating elevates threshold by increasing the noise level of opponent elements, thus reducing the precision with which the visual pathway encodes spatial frequency.