Contrast thresholds of Gabor targets presented after sine-wave grating maskers were measured. All the patterns were isochromatic. The duration of both masker and target was 33 ms. The interstimulus interval was 33 ms. Masker spatial frequency, masker contrast, and target frequency were varied. Two experiments were conducted. In the first there were four target frequencies: 0.5, 1, 2, and 4 c/deg, with five masker frequencies covering a range of +/- 2 octaves around each and five masker contrasts (0.05-0.8). In the second there were three target frequencies (1.5, 2, and 2.83 cycles per degree), five masker frequencies around each, and 13 contrasts (0.005-0.8). Masking functions were bandpass, with maximum masking near the target frequency. Target thresholds increased with masker contrast. Over the shorter, sparser contrast range of experiment 1, functions appear roughly linear or concave upward. When they are plotted on log-log coordinates, functions for different masker frequencies diverge as masker contrast increases. Experiment 2 makes it apparent that some of these functions contain two increasing segments with a flat segment between them. These results are well described by a theory postulating that detection of these targets is mediated by two or three mechanisms that are differentially tuned to spatial frequency. The effect of the forward masker is briefly to desensitize the mechanisms that respond to it. The theory is fitted to the results of experiment 2, and its parameters are estimated.