Human scalp hair is increasingly regarded as a valuable indicator tissue for biological monitoring of environmental exposure to toxic substances. Hair provides both current and past records of exposure during prolonged periods of time. To validate hair monitoring for assessment of toxic substances in space, a unique biological model was developed. Human scalp grafts were transplanted to athymic BALB/c-nu/nu nude mice and then animals were exposed continuously over 2 months, using implanted osmotic pumps, to methylmercury (MeHg), a substance known to be incorporated into hair. Mercury concentrations in hairs were determined using X-ray fluorescence (XRF) spectrometry by segmental analysis of single strands. Blood, skin and brain concentrations of methylmercury were measured by cold vapor analysis. Human scalp hair grown in nude mice showed long-term persistence of human characteristics. Methylmercury was rapidly and stably incorporated into newly formed hair at concentrations that were proportional to the simultaneous concentrations in blood. Hair uptake of methylmercury was a dose-dependent process. This model demonstrates that hairs reflect the level of exposure and that hair analysis can be applied for monitoring of toxic agents during extended space flight.