The gastrointestinal (GI) system is vital in its capacities for nutrient and water uptake, immune function, metabolism and detoxification, and stem-cell derived regeneration. Of significance to human health are a myriad of GI disorders associated with aging that integrate with the circadian clock. Here we present data from three groups of mice: young (3 mo old), middle aged (12 mo old), and old aged (24 mo old). Small intestine and colon samples taken every 4 h under light-dark (LD) conditions were assayed for gene expression related to molecular circadian rhythmicity, transcription, cell signaling, and immune function. Transcripts related to melatonin biosynthesis and signaling, as well as melatonin content from stool, were also included, as GI melatonin and aging have been associated in contexts outside of the circadian clock. With respect to circadian genes, the data here are congruent with data from other peripheral tissues: age does not affect the rhythmic expression of core clock genes in the gut. The same can be said for several clock-controlled transcripts. In contrast, diurnal patterns in the expression of nitric oxide synthase 1 and of immune factors irak4 and interleukin-8 were observed in the colon of young mice that were lost in middle-aged and aged animals. Furthermore, the diurnal pattern of melatonin synthesis genes was altered by age, and stool melatonin levels showed significant decline between young mice and aged cohorts. These data expand the evidence for the persistence of the circadian clock throughout the aging process and highlight its importance to health.