Strain differences have been invoked to explain differing results when studying neural-immune interactions in laboratory animals. We investigated the splenic norepinephrine (NE) content and concentration in three strains of male mice (BALB/C, C57BL/6, and DBA/2), as well as possible diurnal variability in this innervation. Diurnal plasma corticosterone levels served as a positive control. Mice were housed on a 12 h on/12 h off light/dark cycle for 3 weeks, then sacrificed at one of six times during the 24 h cycle. Spleen NE total content and concentrations were determined using high performance liquid chromatography with electrochemical detection. We found small but significant differences between strains in total resting spleen NE content (BALB/C > C57BL/6 > DBA/2) and in resting NE concentration (C57BL/6 > BALB/C > DBA/2). This may reflect differences in spleen weight (BALB/C > DBA/2 > C57BL/6). The expected diurnal pattern of plasma corticosterone was seen in all strains, but no diurnal differences were found in NE content or concentration.