Anabolic steroids (ABS) boldenone (BL; mg/kg) and stanozolol (ST; mg/kg) were administered . to horses and the plasma samples collected up to 64 days. Anabolic steroids and androgenic steroids (ANS) in plasma were quantified using liquid chromatography-tandem mass spectrometry (LC-MS/MS). The limit of detection of all analytes was 25 pg/mL. The median absorption (t1/2 partial differential) and elimination (t1/2e) half-lives for BL were h and h, respectively, and the area under the plasma concentration-time curve (AUCho) was /mL. The median t1/2e for ST was h and the was /mL. Peak mean (X+/-SD) plasma concentrations (Cmax) for BL and ST were and pg/mL, respectively. Quantifiable concentrations of ABS and ANS were found in % of the 988 plasma samples tested from race tracks. In % of the plasma samples two or more ABS or ANS were quantifiable. Testosterone (TES) concentrations mean (X+/-SE) in racing and nonracing intact males were +/- and +/- pg/mL, respectively. TES was not quantified in nonracing geldings and female horses, but was in racing females and geldings. Plasma concentrations of endogenous 19-nortestosterone (nandrolone; NA) from racing and nonracing males were +/- and +/- pg/mL, respectively.