The effects of substance P and the selective neurokinin-1 receptor antagonist (+/-)-CP-96,345 have been compared on in vitro spinal cord preparations from the rat and the gerbil. Substance P produced a concentration-dependent depolarization of motoneurons recorded from ventral roots of both species. The EC50 values (microM mean +/- S.E.M.) obtained in rat (0.95 + 1.0/-0.49) and gerbil (0.47 + 0.26/-0.17) preparations were comparable. The mean maximal depolarization (mV mean +/- S.E.M.) evoked in rat (2.07 + 0.26/-0.25) was approximately two-fold greater than that evoked in gerbil (1.21 + 0.15/-0.14) preparations. In the rat substance P had a biphasic effect (depression followed by potentiation) on the short latency probably monosynaptic reflex evoked by electrical stimulation of a dorsal root. In gerbil preparations substance P produced only potentiation of the monosynaptic reflex. The EC50 values (microM) mean +/- S.E.M.) for this potentiating action in rat (0.97 + 0.75/-0.43) and gerbil (0.46 + 3.6/-0.4) preparations were similar. This potentiation demonstrates a positive modulation of an endogenous excitatory probably glutamatergic transmission by substance P in the ventral horn of the spinal cord. The depressant phase observed in rat preparations may be related to the relative immaturity of myelination in rat ventral root fibres compared to the gerbil. The selective neurokinin-1 antagonist (+/-)-CP-96,345 was one hundred-fold less potent as an antagonist of substance P-induced depolarizations in the rat (pA2 4.69 +/- 0.18, n = 7) than in the gerbil (pA2 6.79 +/- 0.16, n = 5) spinal cord. This finding suggests that (+/-)-CP-96,345 may not act solely at the neurokinin-1 recognition site. In conclusion this study demonstrates that substance P modulates the monosynaptic reflex in the spinal cord presumably via activation of neurokinin-1 receptors.