Delta sleep-inducing peptide-like immunoreactivity (DSIP-LI) was measured by radioimmunoassay (RIA) in individual brain areas of rats running a 12-choice maze. One of the areas found to contain a high level of DSIP-LI was the thalamus, where a distinct peak of immunoreactivity eluted at the position of the synthetic DSIP nonapeptide after high performance liquid chromatography (HPLC). On each of the two days of testing, DSIP-LI in the thalamus correlated significantly with running times in the maze. The results suggest an association of behavior with the levels of peptide in a region of the brain.