1. The elevation of the thresholds for sinusoidal gratings of 4 c/deg and 6.7 c/deg was examined after adapting to gratings of 4 c/deg. Threshold elevation was determined as a function of the adapting contrast.2. It was confirmed that, when the adapting and testing spatial-frequencies were the same, the threshold elevation curve extrapolated to zero elevation at an adapting contrast equal to the pre-adaptation threshold for the adapting grating.3. It was found that the threshold elevation curve for 6.7 c/deg gratings also extrapolated to zero elevation at the pre-adaptation threshold for the adapting grating, even though the testing and adapting frequencies were different.4. It is argued that the latter result shows that adaptation is not simply an after-effect of prolonged excitation of a channel. Threshold elevation curves may represent the bandwidths of inhibitory interactions between channels.