The effects of stimulation of the lateral pterygoid (LP) nerve on the contralateral LP motoneurons (Mns) were investigated in the decerebrate guinea pig. 1. Stimulation of the LP nerve induced a hyperpolarizing potential (HP) in the contralateral LP Mn. The onset and peak latency of this HP were 2.14 ms and 4.10 ms in 11 LP Mns on the average, respectively. The HP was reversed to a depolarizing potential after intracellular Cl- injection, indicating that it mainly consisted of IPSPs. 2. The antidromically evoked field potential (AFP) in the LP Mn pool was depressed by the stimulation of the contralateral LP nerve. The threshold intensity was about 1.2 times the LP nerve threshold. 3. The crossed depression of AFP was significantly diminished after the destruction of the caudal part of the contralateral trigeminal mesencephalic nucleus, but not by the lesion of the trigeminal spinal tract. 4. The most rostral level in the trigeminal spinal tract nucleus, where the HRP-labelled axon terminals were found after its injection into the LP muscle, was located caudally to the lesion of the spinal tract. It is concluded that the muscle spindle afferents from the LP muscle are involved in the crossed inhibition of the LP Mns evoked by the stimulation of the LP nerve.