Physiopharmacological properties of the pupillary dilation produced by the humoral factor in the reflex pupillary dilation were studied in cats anesthetized with ketamine-HC1. Pupillary dilation produced by the humoral factor was separated from that produced by the neural mechanism, ie, sympathetic activation and parasympathetic inhibition, by atropinization of acutely sympathectomized pupil. To evoke the pupillary reflex dilation, the sciatic nerve was stimulated by using a bipolar electrode. The stimulus consisted of trains of 0.5 milliseconds duration rectangular pulses and was given for 5 seconds. The stimulus frequency was usually 50 Hz; high frequency stimuli were more effective to evoke pupillary dilation produced by the humoral factor. The pupillary dilation was recorded with an infrared pupillo-analyzing system. A long latency (about 6 seconds) and long peak latency (about 10 seconds) pupillary dilation was produced by the humoral factor. The threshold of pupillary dilation produced by the humoral factor was always higher than that produced by parasympathetic inhibition. The pupillary dilation produced by the humoral factor was not affected by local application and intravitreal injection of 10(-2) M propranolol-HC1 and 10(-2) M yohimbine-HC1. However, the dilation was markedly inhibited by intravitreal injection of 10(-2) M phenoxybenzamine-HC1 or local application of 10(-2) M bunazosin-HC1.