Perceptual and cognitive influences on line bisection were isolated using a tachistoscopic forced-choice paradigm. Pretransected lines were presented for 150 msec at four orientations ([symbol: see text],-,/ and [symbol: see text]). Subjects made either 'left-right' (for -,/, and [symbol: see text] lines) or 'above-below' (for [symbol: see text],/and [symbol: see text] lines) discriminations in response to each line stimulus, depending upon perceived transector location relative to veridical line midpoint. Median response time and point of subjective equality (P.S.E.) were computed for each treatment condition. P.S.E.s in 'left-right' conditions were significantly left of veridical; response time maxima were similarly displaced. Azimuthal pseudoneglect was greatest for horizontal lines. P.S.E.s in 'above-below' conditions were displaced above veridical, and response time maxima were similarly displaced. Altitudinal pseudoneglect was greatest for negative diagonal lines [symbol: see text]. Azimuthal pseudoneglect significantly exceeded altitudinal pseudoneglect. 'Left-right' responses (mean = 478.3 msec) were significantly faster than 'above-below' responses (mean = 504.6 msec). We conclude that scanning eye and/or gross limb movements do not account for pseudoneglect, and that a significant component must be purely perceptual. Chronometric and psychometric measures of pseudoneglect are in remarkable agreement. The effects of altitudinal and azimuthal pseudoneglect are neither separable nor additive, suggesting the existence of independent mechanisms governing the allocation of spatial attention to objects of differing orientation. The slopes of the psychometric functions for lines of cardinal orientation are significantly steeper than for diagonal lines, which may reflect a processing conflict between these putatively independent mechanisms at diagonal line orientations. Decision context significantly modulates the magnitude of pseudoneglect for physically identical stimuli, perhaps reflecting the selective differential engagement of the vertical or horizontal attentional mechanisms. There are significant individual differences in line bisection performance, even in a very homogeneous sample of strongly right-handed subjects.