Trenbolone (17β-hydroxy-4, 9, 11-trien-3-one) is a synthetic androgenic steroid hormone currently used as a growth promoter for farm animals in several meat exporting countries. Trenbolone is administered in its acetate form (TBA) through a subcutaneous implant and hydrolyzed in the body to the 17β-trenbolone, which is the active hormone and potent anabolic agent with androgenic, gestagenic and anti-glucocorticoid effects. The average OC-normalized sorption coefficient of 17β-trenbolone is 103.25 L/kg, which is similar to what was previously observed for the natural androgen testosterone. Aerobic degradation of 17β-trenbolone spiked into soil microcosms at either 1 mg/kg or 12 mg/kg was measured at 22±2°C in a silty clay loam and a sandy soil pre-equilibrated at field capacity. 17β-trenbolone degraded to trendione and rates were concentration dependent. At lower spiked concentrations, half life was only a few hours whereas at higher applied concentrations, the half lives approach one day. To assess if the concentration-dependence on rate may be due to conversion of trendione back to trenbolone, similar studies were conducted with trendione at applied concentrations of 3 mg/kg and 6 mg/kg. Only a small amount (<1%) of trenbolone was detected at any time. Trendione half life was 4 to 5 days and did not appear concentration dependent. The slower degradation of trenbolone at higher but nontoxic levels may be best explained by the formation of non-activated bimolecular complexes (two substrate molecules per active locus of enzyme) as first proposed by Haldane in 1930 and further extended by Fridenwald and Meangwyn-Davies in 1954. Further elucidation of the concentration–dependent biotransformation of steroids is warranted to better predict dissipation of trenbolone as well as other steroids in fields under manure application.