Graphical Abstract Contamination of waters by 17α-methyltestosterone (MT) has increased in recent years due to the increase in the fish farms production. This work describes the determination of the MT hormone by a new spectrophotometric method in the visible region, after the reaction with potassium nitroprusside with the generation of a compound having maximum absorbance at 400 nm. To understand the process of the compound formation, geometry optimizations were performed at MPW1PW91/6-311+G

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... d,2p) level of theory and the calculations showing that the NO group of nitroprusside bonds to the carbon with double bond adjacent to the carbonyl of 17α-methyltestosterone. In this method, MT was determined at the concentration between 8.0 x 10 -8 and 2.7 x 10 -7 mol L -1 with a coefficient of determination of 0.9946. The relative standard deviation for the repeatability in nine replicates was 2.37% for [MT] = 1.50 x 10 -7 mol L -1 . Limit of detection (LOD) was calculated as 1.75 x 10 -8 mol L -1 and the limit of quantification (LOQ) was established as the lowest value of the linear concentration range (8.0 x 10 -8 mol L -1 ). This spectrophotometric method was suitable according to the analysis of analytical figures of merit, it has low cost and can be applied to samples of fish farm waters with portable spectrophotometers for in situ measurements, increasing the environmental control with rapid analysis.