An original approach is proposed to investigate inorganic (iHg) and methylmercury (MeHg) trophic transfer and fate in a model fish, Danio rerio , by combining natural isotopic fractionation and speciation. Animals were exposed to three different dietary conditions: i. 50 ng Hg g-1, 80% as MeHg; ii. diet enriched in MeHg 10000 ng Hg g-1, 95% as MeHg and iii. diet enriched in iHg 10000 ng Hg g-1, 99% as iHg. Harvested was carried out after 0, 7, 25 and 62 days. Time-dependent Hg species distribution and isotopic fractionation in fish organs (muscle, brain, liver) and faeces, exhibited different patterns, as a consequence of their dissimilar metabolization. The rapid isotopic re-equilibration to the new MeHg-food source reflects its high bioaccumulation rate. Relevant aspects related to Hg excretion are also described. This study confirms Hg isotopic fractionation as a powerful tool to investigate biological processes, although its deconvolution and fully understanding is still a challenge.