Primary sensory information from neurons innervating whisker follicles on one side of a rat's face is relayed primarily through two subnuclei of the brainstem trigeminal complex to the contralateral thalamus. The present experiments were undertaken to separate the contribution of the principal trigeminal nucleus (PrV) from that of the spinal trigeminal nucleus (SpV) to whisker evoked responses in the ventral posterior medial (VPM) nucleus in the adult rat thalamus. Extracellular single-unit responses of VPM neurons to controlled stimulation of the contralateral whiskers under urethane anesthesia were quantified in terms of receptive field size, modal latency, response probability and response magnitude. The SpV contribution to VPM cell responses was isolated by making kainic acid lesions of the PrV. The PrV contribution was ascertained by cutting the trigeminothalamic axons arising from SpV just before they cross the midline. After destruction of the PrV, the SpV pathway alone produced large receptive fields (mean: 9.04 whiskers) and long latency (mean: 11.07 ms) responses from VPM neurons. In contrast, PrV input alone (SpV disconnected) generated small receptive fields (mean: 1.06 whiskers) and shorter latency (mean: 6.74 ms) responses. With both pathways intact the average receptive field size was 2.4 whiskers and peak (modal) response latency was 7.33 ms. The responses with both pathways intact were significantly different from either pathway operating in isolation. Response probability and magnitude followed the same trend. We conclude that normal responses of individual VPM neurons represent the integration of input activity transmitted through both PrV and SpV pathways.