Leptin directly influenced testosterone secretion by the testes in vitro, and MSG had no obvious effects. Equal decapsulated testes were incubated with several concentrations of leptin, and the testosterone of the supernatant was detected by EIA. In low concentrations (<10 nM), leptin stimulated testosterone secretion, and high concentrations (>10 nM) of leptin inhibited this secretion, regardless of the age of the mice (A). Testosterone secretion had no significant variations when the testes were stimulated with different concentrations of MSG in vitro (B). Statistical method was performed by one-way analysis of variance. *, p < 0.05 compared with other concentrations; NSD, non-significant difference compared with other concentrations.