Excitatory receptive field (ERF) response profiles and length summation functions were derived from complex neurones in cat striate cortex. Measured length summation was compared with summation predicted from integration over ERF profiles. In a minority of neurones, measured and predicted summation were well matched. In the majority, whether end-stopped or not, responsiveness in length summation tests was appreciably greater than predicted for short stimuli, compared with ERF profiles. The mismatch was least in standard and greatest in special complex neurones; in the latter group, response levels to long stimuli fell well below predicted levels. In end-stopped neurones the decremental portion of length summation functions was not predicted by ERF profiles. These results implicate the involvement of non-linear mechanisms, whereby concomitant stimulation of central regions of the receptive field (RF) potentiate the efficacy of loci towards either end of the RF.