Retinitis pigmentosa (RP) is an inherited retinal disorder clinically characterized by a pale, waxy optic nerve head, attenuated retinal blood vessels and bone spicule pigment in the retina. Hemodynamic studies have demonstrated that RP is associated with a reduction in the retinal and choroidal blood flow. Retinal hemodynamic impairment is also present in the early stages of RP and various hypotheses have been advanced as to its cause. The authors studied 20 patients, 12 males and 8 females, aged between 26 and 42 years (mean 34.6 years) affected by simplex RP. The twenty patients were divided in two groups according to the degree of sight impairment: group A consisted of 10 patients with a visual acuity of 0.3 ± 0.1, visual field mean defect 18.988 ± 3.419 dB and b-wave electroretinogram amplitude of 13.14 ± 0.308 µV. Group B consisted of 10 patients with a visual acuity of 0.8 ± 0.2, visual field MD 10.523 ± 3.582 dB and b-wave electroretinogram amplitude of 26.000 ± 0.757 µV. An increase in plasma levels of endothelin-1 (ET-1) was found as compared with healthy controls: 1.910 ± 0.617 pg/ml vs. 1.180 ± 0.210 pg/ml (p < 0.021), but there was no statistical difference between group A and B (p < 0.163). It is thought that an increase in ET-1 and retinal oxygen levels in RP could lead to vasoconstriction and a decrease in the retinal blood flow worsening the abiotrophic process.