Changes in circulating concentrations of testosterone and estrone sulfate after human chorionic gonadotropin administration and subsequent to castration of 2-year-old stallions

•Testosterone response to hCG in two-year-old stallions.•Testosterone response to hCG had a maximal increase at 48, 72 and 96 h post-hCG.•There was no change in estrone sulfate concentrations after hCG.•After castration, half-lives of testosterone and estrone sulfate were 1.1 and 0.7 h, respectively.•Both hormones were not significantly different at 24 h versus 15 days post-castration.