Birds’ seasonal transitions between annual cycle stages are regulated by several interacting hormones inducing physiological changes. In spring, lengthening photoperiod induces the expression of deiodinase enzyme in birds’ hypothalamus, transforming the thyroid hormone T4 into T3. In turn, T3 is thought to facilitate the release of gonadotropin-releasing hormones (GnRH) by acting at the median eminence. This activation of the hypothalamic-pituitary-gonad (HPG) axis results in gonad maturation and increases circulating sex steroid hormones. Environmental stressors such as exposure to methylmercury (MeHg) contamination could block deiodinase enzyme actions and lead to inhibition of GnRH synthesis or secretion. Such GnRH disruption could strongly impact avian seasonal timing, physiological adjustment and reproductive output. We experimentally determined if MeHg can affect songbirds’ seasonal timing through disruption of endocrine balance during the transition to spring phenotype. We captured male song sparrows (Melospiza melodia) and exposed them to an environmentally relevant dose of MeHg for 90 days before photostimulation, and kept them for post-exposure observations for 23 days. Preliminary results indicate that testis volume (measured post-mortem) was not affected by MeHg exposure. We will present results of thyroid hormones and testosterone levels measured throughout this experiment.