Blood pressure (BP) is a crucial physiological parameter that exhibits a circadian rhythm in healthy individuals. Loss of this rhythm is associated with morbidity and mortality. To determine the contribution of the molecular clock to BP regulation, we developed a rat strain with whole body loss of Bmal1 gene expression,a core component of the transcription/translation feedback loop known as the molecular clock. We previously reported that Bmal1-/- rats of both sexes have rhythmic BP, but lower than Bmal1+/+ controls. Given the importance of the renin-angiotensin system in BP control, we hypothesized that loss of Bmal1 may reduce plasma renin activity in the Bmal1-/- rat. For this experiment, male rats were housed in a 12:12h light/dark cycle with constant humidity and temperature. Water and food (0.49% NaCl) were available ad libitum. BP was measured by a telemetry transmitter implanted in the abdominal aorta of 10-week-old rats. After a 10-day surgical recovery period, BP was recorded for 2 minutes every 10 minutes at 1000 samples/second. Mean BP, assessed by cosinor analysis, was lower in male Bmal1-/- (MBP MESOR: 109±2 mmHg, n=9) animals compared to littermate Bmal1+/+ controls (MBP MESOR: 117±3 mmHg, n=8, p<0.05, ANOVA). In a separate cohort of animals, plasma was collected from male Bmal1-/- rats and littermate Bmal1+/+ controls across the circadian day every four hours in groups of 2-6 at each time point starting at zeitgeber time (ZT) 0 (07:00) and ending at ZT 20. Plasma renin activity (PRA) and total prorenin/renin were measured by ELISA (name the kits used). PRA was similar between male Bmal1-/- and Bmal1+/+ rats during the light (Bmal1+/+ : 1.4±0.5 vs Bmal1-/- : 2.5±0.6 ng/mL, p=0.21, Student's t-test) and dark (Bmal1+/+ :2.4±0.7 vs Bmal1-/- : 2.4±0.5 ng/mL, p=0.97, Student's t-test) periods. Plasma total prorenin/renin was similar between Bmal1-/- and Bmal1+/+ rats during the light (Bmal1+/+ : 8.8±1.7 vs Bmal1-/- : 6.9±0.7 ng/mL, p=0.36, Student's t-test) and dark (Bmal1+/+ : 2.7±1.3 vs Bmal1-/- : 5.2±1.7 ng/mL, p=0.27, Student's t-test) periods. These data suggest that loss of Bmal1 does not directly affect systemic plasma renin activity or total prorenin/renin concentrations in the rat. Furthermore, the lowered blood pressure observed in rats lacking Bmal1 does not appear to be a result of systemic changes in circulating renin activity or concentration.