In the present study, we have shown by single afferent unit recording in the organs of Lorenzini that L-serine-o-phosphate (L-SOP) decreases the resting discharge frequency as well as electrically evoked responses. L-SOP in a concentration of 0.1–100$mUM antagonizes responses induced by L-glutamate (L-GLU) andN-methyl-D-aspartate (NMDA) and has no effect on the responses to application of kainate (KA) and quisqualate (Q). The results obtained confirm previous observations about the existence of NMDA receptors in the afferent synapse of the ampullae of Lorenzini.