The retinal vessel calibre responses to systemic autonomic stimulation were studied in fellow eyes of 11 patients with central retinal vein occlusion (CRVO) and 10 patients with branch retinal vein occlusion (BRVO), using sustained isometric muscle contraction as the stimulus. These vessel responses were compared to those of a control group of 11 subjects of similar ages. The changes in retinal vessel calibre were measured using the Quantimet Image Analyser. There was no significant difference in mean arteriolar constriction during isometric muscle contraction (mean +/- SEM) between the CRVO group (6.0 +/- 1.34%) and the BRVO group (5.4 +/- 0.31%), (p greater than 0.05), and between either of these groups compared with the control group (7.4 +/- 1.20%), (p greater than 0.50). Similarly there was no significant difference in mean venule responses between the groups (CRVO 4.9 +/- 0.48%; BRVO 4.6 +/- 0.63%; control subjects 3.8 +/- 0.90%; p greater than 0.05). The diastolic blood pressure responses were similarly not significantly different between the 3 groups (CRVO 22.3 +/- 1.27, range + 17 to 29 mmHg; BRVO 25.8 +/- 2.9 mmHg, range 18 to 45 mmHg; control subjects 21.3 +/- 1.7 mmHg, range 16 to 35 mmHg), (p greater than 0.10). The implications of the results are discussed.