Experiments utilizing the retrograde transport of horseradish peroxidase (HRP) were performed in order to locate the cells of origin of ascending projections to the parafascicular nucleus (Pf) of the cat. HRP-labelled cells were identified in several regions of the brain stem including: trigeminal nuclei, vestibular nuclei, nucleus coeruleus, tegmental field, deep layers of the superior colliculus, substantia nigra, dorsal median nucleus of Raphe and periaqueductal grey substance. Of these, the periaqueductal grey contained approximately 40% of labelled neurones. A weak spinal cord projection to Pf originated bilaterally from laminae VI and VII-VIII at C1 and C2 and some neurones were also found contralaterally at C7 and L7.