The substantia gelatinosa of the mammalian spinal cord is generally believed to be a closed system; that is, its neurons are thought to project only to the substantia gelatinosa of the same or the contralateral side. Experiments in monkeys, using injections of the marker enzyme horseradish peroxidase, show that at least some neurons of the substantia gelatinosa project to the thalamus and thus belong to the spinothalamic tract. Such neurons include two cell types intrinsic to the gelatinosa, the central cells and the limitrophe cells of Cajal.