[show abstract][hide abstract] ABSTRACT: It is well established that the mammalian circadian system consists of pacemaker cells in the suprachiasmatic nuclei (SCN). The mouse has become increasingly important in understanding the circadian timing system, due to the availability of mutant animals with abnormal circadian rhythms. In the present paper, we describe the organization of the mouse SCN, comparing the wild type and Clock mutant animal, with a special focus on those peptides bearing an upstream E-box element (vasopressin, vasoactive intestinal peptide, cholecystokinin and substance P). To this end, we describe the distribution of the foregoing SCN peptidergic cell types as well as gastrin-related peptide, calretinin, calbindin, somatostatin, neurotensin and retinal input to the SCN (determined by both tract tracing and fos-immunoreactivity in response to a light pulse). The Clock mutant mouse has decreased expression of vasopressin mRNA and protein in the SCN, with normal patterns of expression elsewhere in the brain. No other differences were detected between the Clock mutant and the wild type mouse. The results are consistent with the hypothesis that there are multiple regulatory elements of clock-controlled genes in the SCN.