Precis: After a short-term intraocular pressure (IOP) elevation, the central retinal vein caliber may be widened at lower IOP rise levels, while be compressed at higher IOP rise values. Purpose: The purpose of this study was to investigate changes in the calibers of the central retinal vein trunk (CRVT) and central retinal artery trunk (CRAT) trunk during a short-term elevation of IOP. Methods: A prospective observational study. Acute primary angle-closure suspects underwent a dark room prone provocative test (DRPPT) for 2 hours. Before and at the end of the test, tonometry, swept-source optical coherence tomography, and nonmydriatic fundus photography were performed. The calibers of the CRVT and CRAT were measured on the fundus photos taken at baseline and at the end of the DRPPT. Results: The study included 101 eyes (61 individuals; mean age: 54.8 +/- 9.3 y; range: 30 to 70 y) which showed an increase in IOP by 9.6 +/- 9.0 mm Hg (range: 2.3 to 46.7 mm Hg). From baseline to the end of the DRPPT, the mean CRVT caliber increased from 101.8 +/- 25.9 to 107.7 +/- 26.6 mu m (P<0.001), while the CRAT caliber did not differ significantly (110.3 +/- 24.2 vs. 109.7 +/- 21.5 mu m;P=0.54) during the test. The CRVT widening was larger in the subgroup with IOP rise of <6 mm Hg than in the subgroup with an IOP rise of 6 to 15 mm Hg, while in the subgroup with an IOP rise of >15 mm Hg the CRVT caliber did not change significantly (P=0.20) during the test. Conclusions: A physiological short-term IOP rise at lower levels of IOP elevation led to a widening of the CRVT, while at higher IOP values, the further IOP-rise may have compressed the retinal vein. Because of higher intraluminal pressure values, the retinal artery diameters were not affected by the IOP-rise.