The concentrations of vasoactive intestinal polypeptide, somatostatin and substance P were determined in various cerebral subdivisions of adult and foetal Japanese monkeys (Macaca fuscata fuscata) by specific radioimmunoassays. In adult tissues, the highest level of vasoactive intestinal polypeptide was found in the somatosensory cortex and the lowest level in the occipital cortex. A high level of somatostatin was found in the association cortex (prefrontal, parietal and temporal cortex); the lowest level was noted in the occipital cortex. Substance P was found to be high in prefrontal and temporal cortex. The highest levels of substance P and somatostatin were obtained in the amygdala. Between embryonic 4 and 5.5 months, concentrations of peptides increased dramatically, and in the adult, all neuropeptides in cortical subdivisions significantly decreased. By the gel filtration method, only one immunoreactivity which coeluted with substance P and vasoactive intestinal polypeptide was demonstrated in extracts of 4-, 5.5-month-old and adult monkey cerebral cortex. In contrast, somatostatin immunoreactivity eluted as 3 peaks. Almost 80% of the immunoreactivity co-eluted with synthetic somatostatin, regardless of the age of the tissue. The molecular weights of two larger molecules were determined to be 13 and 3 kdaltons.