In this mainly chemical study it is shown that the miotic activity of physostigmine (eserine), which occurs in ripe seed of Physostigma veneno-sum Balfour, is due mainly to the urethane grouping in the molecule. Of the active urethanes, here synthesised, those which, like physostigmine, are derivatives of methyl-carbamic acid appear to be the most active miotics. Certain correlations established between the chemical structure of the synthetic urethanes, and the effect on the iris suggest that the miotic activity of these substances is chemical rather than physical.