With the indirect immunofluorescence technique of Coons, the overall distribution of the substance P (SP)-positive neuron system in the frog brain and spinal cord was explored. SP-positive cells were observed in more than ten areas, such as olfactory bulb, amygdaloid complex, septal area, bed nucleus of hippocampal commissure, hypothalamic periventricular zone, dorsal and ventral thalamus, infundibulum, torus semicircularis, optic tectum, the area dorsal to the interpeduncular nucleus, central gray matter of the mesorhombencephalon, and raphe region, etc. A dense network of SP-positive fibers was also widely distributed in the frog brain and spinal cord. SP-positive fibers were roughly divided into two types. One consisted of very fine SP-positive fibers and gave the region a diffuse appearance. The area medial to n. Bellonci, interpeduncular nucleus, n. isthmi, and optic tectum contained this type of SP-positive fibers. The other one consisted of clearly distinguishable varicose fibers. A number of SP-positive fibers located in the amygdaloid complex, striatal complex, hypothalamus, central gray matter of the mesorhombencephalon, trigeminal spinal nucleus, and posterior horn of the spinal cord belonged to this category. The functional role of the SP-positive neuron system in the central nervous system is also briefly discussed.