Abstract Body: Kidney function follows a distinct time-of-day rhythm that persists independently of external cues like light exposure, sleep, physical activity, or meal timing. Recent research has shown that this diurnal variation is driven by an internal molecular clock within the kidneys themselves. We observed that loss of this transcriptional regulatory system impairs functional diurnal rhythms in glomerular filtration rate (GFR) and sodium excretion (UNaV) in a novel brain and muscle ARNT-like protein 1 (Bmal1; a core component of the molecular clock) knockout (KO) rat in a sex-dependent manner. We found that male, but not female Bmal1 KO rats had loss of diurnal variation in both GFR and UNaV. We hypothesized that this was due to reductions in tryptophan (Trp) bioavailability, since our preliminary studies found Bmal1 KO males have significantly lower plasma Trp concentrations compared to wildtype littermate controls (WT) and the well known effects Trp on GFR. We performed a targeted metabolomics analysis of WT and Bmal1 KO male and female rats plasma at two times of the day using an LC/MS panel containing 9 metabolites (all data reported in ng/mL) spanning the three most common Trp metabolic pathways; 1) kynurenine (KYN), 2) indole, and 3) serotonin. We found that in the KYN pathway, KYN itself was significantly lower in both Bmal1 KO males (46.7±1.7 vs 32.7±1.9; WT vs Bmal1) and females (36.0±2.3 vs 20.7±1.8; WT vs Bmal1). The downstream metabolite picolinic acid was also significantly lower but only in Bmal1 KO females (13.1±0.5 vs 10.0±0.6; WT vs Bmal1). The indole pathway showed similar sex-specific results with both tryptamine (2.8±0.02 vs 2.6±0.02; WT vs Bmal1) and indole-3-acetate (54.6±1.9 vs 36.1±2.6; WT vs Bmal1) being significantly lower only in Bmal1 KO females. To determine whether testosterone was responsible for these sex-specific differences, we studied a sub-group of WT males following orchiectomy (ORX). Surprisingly, post-ORX tryptamine (2.8±0.02 vs 3.0±0.03; Sham vs ORX), indole-3-acetate (52.3±4.4 vs 67.8±3.3; Sham vs ORX), and picolinic acid (13.2±0.6 vs 16.0±0.8; Sham vs ORX) were all significantly higher. These results suggest that Trp metabolism is differentially regulated by both sex and genotype in Bmal1 KO rats. This work further supports our hypothesis that the molecular clock controls GFR through regulation of Trp metabolism.