Immunoreactive delta sleep-inducing peptide (DSIP) is known to occur in the central nervous system and in body fluids including cerebrospinal fluid, blood and urine. However, the exact nature of the immunoreactive material demonstrated has been a matter of discussion. In the present study, DSIP-like immunoreactivity was demonstrated in the porcine pituitary and adrenal medulla using radioimmunoassay and immunocytochemistry. In the pituitary, the DSIP-like material was present in a subpopulation of the cells storing adrenocorticotropin and α-melanotropin. The DSIP immunoreactive cells in the adrenal medulla were identical with a major population of the noradrenaline-storing cells. High-performance liquid chromatography of tissue extracts revealed one major peak of DSIP-like immunoreactivity which did not coelute with synthetic DSIP. The immunoreactive material may represent N-terminally truncated fragments of DSIP. The present results suggest that the anterior and intermediate lobes of the pituitary and the adrenal medulla are potential sources of DSIP-like peptides