It has been reported that the tuning function relating masking of a vertical line to the orientation of a masking line differs in form depending upon whether the mask is presented with the target (simultaneous masking) or before it (forward masking). From such evidence, it might be concluded that different mechanisms are implicated. Lovegrove (1976) suggested that lateral inhibition mediates the simultaneous effect, whereas postexcitatory suppression (adaptation) mediates the successive effect. The six experiments reported here show that apparent motion cues reduce the effectiveness of successive masking when single-line stimuli are used, but that when such motion cues are eliminated, the two tuning functions are similar in form. It is, thus, unnecessary to invoke separate mechanisms in the two cases.