We have examined two models for the preference displayed by cortical simple cells for elongated stimuli having a particular orientation. Both assume that geniculate afferents with aligned receptive fields pool to form the receptive field of the cortical unit. The first model [Marr and Hildreth, Proc. R. Soc. Lond. Ser. B 200, 269-294 (1980)], includes AND gating along the length axis so that a simple cell does not fire unless a critical number of its afferents with adjacent receptive fields are firing. The second model assumes that geniculate input is simply summed over subunits and then passed through a firing threshold. Both models account for the unresponsiveness of simple cells to spots of light, but the AND model predicts a discontinuous length threshold, while the summation model predicts that length and contrast should be interchangeable in the determination of the response threshold. Experiments in which length and contrast were systematically varied support the summation model, and extend the notion of linear spatial summation to the length axis in simple cells.