AIM: To evaluate the ocular outcomes and to elucidate possible mechanisms underlying intraocular pressure (IOP) change following the head-down tilt (HDT) test. METHODS: The study included 21 participants at the Department of Ophthalmology of Tongji Hospital. Subjects received the test of I-care tonometry, enhanced depth imaging optical coherence tomography and heart rate variability (HRV) analysis before and after 15min HDT test. The lumen area of Schlemm's canal (SCAR), IOP, HRV were calculated. RESULTS: IOP increased significantly after 20 degrees head down position from 14.0 +/- 3.0 to 17.0 +/- 3.3 mm Hg (P<0.001). SCAR decreased from 13449.0 +/- 5454.9 mu m(2) at sitting condition to 9576.6 +/- 4130.9 mu m(2) post 15min HDT test. High frequency (HF) indices increased significantly from 1462 +/- 865 Hz at baseline to 2128 +/- 824 Hz. Heart rate (HR) decreased significantly from 76 +/- 11.48 to 70 +/- 11.52 bpm after the HDT. The linear regression analysis showed that the difference of HF and SCAR significantly correlated with each other during the HDT (R-2=20%, P=0.04). CONCLUSION: These outcomes perform the first evidence of the activation of autonomic nervous system of HDT may cause the collapse of Schlemm's canal lumen, which in turn leading to the increased IOP.