Abstract The effects of bath perfusion of polyclonal and monoclonal antibodies to substance P (SP) on slow excitatory transmission in rat dorsal horn have been investigated by intracellular recording in the immature rat spinal cord slice preparation. Both polyclonal and monoclonal antibodies to SP produced a significant decreasesin the amplitude and the duration of the slow depolarization generated in dorsal horn neurons by high intensity, repetitive dorsal root stimulation or exogenous SP application. The effect of endogenous SP, or SP-related peptide, released during dorsal root stimulation appears likely since bath perfusion of a slice with a normal rabbit serum, or affinity chromatography preadsorbed SP antiserum, or non-specific IgG, or 5-hydroxytryptamine antiserum had no similar depressant effect. These results, if taken together with other experimental evidence, suggest that SP, or SP-like peptide, is in some way involved in a generation of the dorsal root-evoked slow depolarization. In addition, a novel approach is presented for using polyclonal and monoclonal antibodies to SP as pharmacological antagonists. Use of a specific characterized monoclonal antibody for the detection of physiological and pharmacological effects of putative peptide transmitters in vitro opens new avenues for further investigations.