The effect of intravitreally injected colchicine (100 mug and 300 mug) and vinblastine (100 mug) on function and sensitivity of the cat iris was studied in vivo. Both antimitotics caused the same effects: The sensitivity to touch of the cornea disappeared after the third day. The normal sensitivity recovered, but after 1 month there was still an anesthetic island in the center of the cornea. The consensual light reflex from the injected to the normal eye declined markedly between 12 and 24 h and was abolished after 4 days. The consensual light reflex from the normal to the injected eye decreased with a similar time course and was completely abolished or strongly reduced after 4-5 days. The function of the sphincter pupillae in the injected eye was reestablished after one month. The affected iris became supersensitive to both pilocarpine and norepinephrine. The degree of supersensitivity in the cholinergic system was closely and inversely related to the degree of impaired function. The function of the dilatator pupillae could not be tested. The observed phenomena were most likely due to inhibition of exoplasmic flow in the nerves supplying the iris and cornea.