This study is concerned with midbrain influences on the transmission of thermal information from the rat's scrotal skin to the specific thalamus. Single unit recordings were made from neurons in the ventrobasal thalamus which responded to scrotal warming. After a thermoresponsive neuron had been identified, a small area of the medial midbrain was electrolytically lesioned and the thermal response of the unit was tested again. Lesioning the midbrain raphe nuclei, centralis or dorsalis, or parts of the central grey matter was always followed by a complete loss of thermal responsivity. Lesions which destroyed the rostral end of the nucleus raphe centralis together with parts of the adjacent reticular formation had no effect on thalamic discharge rates. It is concluded that the midbrain raphe nuclei and the central grey matter form an essential part of the extralemniscal pathways which transmit peripheral thermal information to the rat's thalamus.