For the evaluation of a possible malfunction of the blood-retinal (BRB) and the blood-aqueous barrier (BAB) in type I diabetes without manifest angiopathy, after i.v. injection of sodium fluorescein, the permeability of BRB (P) and the diffusion coefficient of BAB [P(a)] were studied simultaneously by fluorophotometry in 34 eyes of 34 type I diabetics (HbA1c = 6.6 +/- 0.9%) without retinopathy whose ages ranged from 19 to 38 years (30.5 +/- 5); diabetes' duration was between 5 and 18 years. Fluorescein angiography was performed to exclude nonperfused areas. In all, 34 eyes of 34 healthy volunteers whose ages ranged between 23 and 34 years (28.5 +/- 3.3) served as controls; in this group, fluorophotometry was performed twice to evaluate reproducibility. The mean BAB diffusion coefficient in diabetics [P(a) = 5.3 +/- 1.8/min] was significantly increased (p = 0.00003) as compared to controls [P(a) = 3.7 +/- 0.7/min]; BRB permeability in diabetes (P = 3.2 +/- 1.4 x 10(-7) cm/s) was raised with this elevation being of lower significance (p = 0.019; controls: P = 2.6 +/- 0.7 x 10(-7) cm/s). We found a decrease in BRB permeability depending on diabetes' duration (r = -0.15; p = 0.007) that was not significant in BAB (r = -0.1; p = 0.24). No correlation was found to exist between permeability and HbA1c values either in BAB or in BRB. The reproducibility in controls was 9% in BRB determinations and 12% in BAB measurements. These results may suggest that early structural alterations without the manifestation of retinopathy possibly cause elevation in BRB permeability and are even more obvious in BAB permeability. Whereas the reliability of vitreous fluorophotometry in detecting early BRB malfunction has to be judged critically, anterior segment fluorophotometry is a reliable procedure for the monitoring of BAB affection in type I diabetes without retinopathy.