Weed pressure threatens lentil (Lens culinaris Medik.) yields, with metribuzin offering control but risking crop injury. This study used hydroponics to screen metribuzin tolerance in lentils, determining the lethal dose 50% (LD50) for lentil cultivar, CDC Greenstar and profiling metabolites in three genotypes VIR421 (susceptible), CDC Greenstar (tolerant), and NZ2022 (medium-tolerant) via liquid chromatography-mass spectrometry (LC-MS). CDC Greenstar plants in a hydroponic deep-water culture system were exposed to metribuzin doses (0.17, 0.25, 0.51, and 2.05 g a.i. ha⁻¹, plus a control) selected based on preliminary trials that identified the effective range for LD50 estimation in hydroponics, where herbicide bioavailability is higher than in soil due to direct root exposure and absence of soil adsorption. These doses are substantially lower than the recommended field application rate of 205 g a.i. ha⁻¹ as a pre-emergence for lentils to account for the amplified effects in hydroponics for 24 hours, with biomass reductions assessed over 21 days. The LD50 was 0.4407 g a.i. ha⁻¹ (R2 = 0.94), with dose strongly reducing shoot/root growth (r = -0.92 to -0.99). Untargeted LC-MS identified seven metabolites in CDC Greenstar and VIR421, including desamino-metribuzin (DA) and conjugates, while targeted LC-MS tracked metribuzin, DA, and desamino-diketo-metribuzin (DADK) over 12 days. VIR421 had higher metribuzin levels (105.70 ng/g at dry weight at 12 h) compared to CDC Greenstar and NZ2022, which rapidly metabolized it to DA (58 and 50.41 ng/g dry weight at 2 days), with NZ2022 showing further metabolism by 4 days. DA dominated 59 to 167-fold over DADK, suggesting a primary detoxification pathway. Hydroponics enabled precise tolerance screening, revealing genotype-specific metabolism critical for breeding metribuzin-tolerant lentils, enhancing weed management strategies.