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SUMMARY1. Miosis was observed after enucleation in unopened eyes from normal or atropinized, atropinesterase-free rabbits. Such a phenomenon was not seen in enucleated cat eyes, in which the pupils remained widely dilated, whether atropine had been administered or not.2. Pre-treatment of the animals with reserpine did not alter this difference between the species.3. The difference does not appear to be due to absence of irins from the cat iris, since aqueous extracts of cat irides contained a smooth-musclecontracting activity (cat irin) extractable into ether at pH 3 and therefore consisting of lipid acid(s).4. The difference is not due to insensitivity of the cat sphincter pupillae muscle to irins, since injections of ether-purified cat or rabbit irins into the anterior chamber of enucleated cat eyes kept at room temperature constricted the pupil; injections of histamine were ineffective.5. In experiments on animals treated with atropine + mepyramine i.v., photographic measurements revealed a further difference, namely in the speed of miosis after stroking the iris in vivo. The response started later in the cat, and developed more slowly, but often to a fuller extent than in the rabbit.6. In a proportion of cat eyes there was little or no change in intraocular pressure after irritation of the iris adequate to induce maximum pupillary constriction; this was so whether mepyramine had been administered or not.7. Possible reasons for the above species differences are discussed.

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