Novel fluid medium pressure cells were used to deform antigorite under constant stress creep conditions at low temperature, low strain rate (10^-9 - 10^-4 1/s), and high pressure (1 GPa) in a Griggs-type apparatus. Antigorite cores were deformed at constant temperatures between 75°C and 550°C, applying 8-12 stress-strain steps per temperature. The microstructures of deformed samples highlight the importance of basal shear and kinks to antigorite plasticity. Rheological data were fit with a low temperature plasticity law, consistent with a deformation mechanism involving large lattice resistance. When applied at geologic stresses and strain rates, the extrapolated viscosity agrees well with predictions based on subduction zone thermal models.