Using a contrast matching procedure, we measured the perceived contrast of vertical test gratings after adapting to other gratings of either vertical or horizontal orientation. The results show that both parallel and orthogonal adapting gratings reduce perceived contrast and do so proportionally more at low test contrasts than at high. The results are consistent with a single mechanism model proposed by Boss and Speed [(1991). Proceedings of the Royal Society (Series B), 246, 61-69] that assumes that adaptation to gratings repositions contrast-response transducer functions. They are not consistent with the notion of two different forms of adaptation, subtractive for parallel and multiplicative for orthogonal adaptors as proposed by Snowden and Hammett [(1992). Nature, 355, 248-250]. Nowhere is the reduction in perceived contrast by an orthogonal grating greater than that by a parallel grating of the same contrast. A direct comparison using two orthogonal adaptors confirms the greater potency of parallel adaptors, but also reveals interactions between the adaptors. Copyright (C) 1996 Elsevier Science Ltd.