Rodenticides are a necessary part of successful large-scale rodent control programs, but poor bait acceptance by rats compromises the effectiveness of many baiting programs. Non-toxic bait markers are one approach for assessing consumption and identifying the cause of failure. In single-feeding laboratory bioassays, we evaluated metallic flakes, Solvent Blue 36 oil-soluble dye, and tetracycline hydrochloride (THC) as bait markers for monitoring consumption of oat groats by captive black rats (Rattus rattus). We detected metallic flakes (0.3% [g/g] dietary concentration) in the gastrointestinal (GI) tracts of all 12 rats examined immediately following a single 24-hour feeding trial but in only 1 of 12 rats examined 48 hours later. We easily detected Solvent Blue 36 oil-soluble dye (0.1% to 0.5% [g/g]) in subcutaneous, abdominal, and genital fat of 7 of 8 rats 2 days after feeding, but found it in only 6 of 12 rats examined 5 days after feeding. Dye applied at 1.0% and 1.5% (g/g) persisted in all 8 rats inspected 5 days after feeding, but reduced consumption (P < 0.05) at these levels suggests that concentrations ≥1% are unpalatable to rats. Dietary concentrations of THC from 0.5% to 1.0% (g/g) produced a golden-yellow fluorescence under ultraviolet (UV) illumination on the incisors and growing points of the mandibles of all rats examined 3 and 14 days after feeding. Because THC had no apparent effect on consumption (P = 0.28), it offers a reliable means of detecting consumption by black rats and thus, of determining whether poor results with rodenticides are due to poor bait acceptance.