OBJECTIVES: To determine if acupuncture stimulation elicits a pupillary response under light adaptation and whether there is any difference in the pupillary response between light and dark adaptation environments during acupuncture stimulation. METHODS: The participants consisted of 55 healthy individuals who had no known eye diseases or pupil abnormalities. Experiment 1 compared pupillary responses between acupuncture stimulation and no-stimulation groups under light adaptation. Experiment 2 compared pupillary responses to acupuncture between two conditions (dark and light adaptation) with a two-period repeated measurement crossover design. For both experiments the pupil diameter was continuously measured for 3 min before stimulation, during stimulation and for 3 min after stimulation. For all acupuncture stimulation interventions an acupuncture needle was inserted superficially at the TE5 acupuncture point followed by gentle tapping stimulation for 90 s. RESULTS: In experiment 1 the pupil diameter was significantly decreased during (p<0.01) and after stimulation (p<0.0001) compared with the pupil diameter before stimulation under light adaptation. No significant difference was noted in the serial changes in pupil diameter in the no-stimulation group. In experiment 2 the pupil diameter was significantly decreased 90 s after stimulation (p<0.05) and 150 s after stimulation (p<0.05) under light adaptation conditions. Furthermore, the pupil diameter was significantly decreased 120 s after stimulation (p<0.05) and 150 s after stimulation (p<0.01) under dark adaptation conditions. No significant difference in the serial changes in pupil diameter was noted between the groups. CONCLUSIONS: This study shows that pupil constriction occurs following acupuncture stimulation under light adaptation and this response is no different from that seen under dark adaptation.