Tear flow was measured in normal human eyes by means of a radioisotope (technetium as pertechnetate in a normal saline solution), a gamma camera and a computer. By "region of interest" technique the elimination was shown to have two phases: an initial rapid elimination followed by a slower elimination after 5--7 min. The mean fractional turnover rate in the initial phase was 0.197 min.-1 (n = 35, SEM = 0.013) and in the basal phase 0.083 min.-1 (n = 35, SEM = 0.003). Assuming a constant tear volume these values corresponded to a tear flow of 1.4 microliter min.-1 and 0.6 microliter min.-1, respectively. There was no significant difference between the fractional turnover rate in the upright and supine position. No difference was found in tear flow between males and females. With the eyes closed the fractional turnover rate was low with intermittently rapid outflow of tears. Irritation to the contralateral eye with a filterpaper caused a stimulated tear flow of 4.4 microliter min.-1. A nomogram facilitating background corrections was constructed.