The pre-retinal vitreous space of the eye of albino rabbits was perfused in vivo via a dialysis probe in order to measure the concentration of endogenous amino acids. Studies of isotope concentrations in the vitreal effluent following intracarotid injections of 3HOH and [14C]mannitol demonstrated that entrance of 3HOH into the dialysis probe occurred within a few minutes whereas [14C]mannitol was almost completely excluded. Among the amino acids, analysed with high-pressure liquid chromatography (HPLC), glutamine had a concentration similar to that in plasma and cerebrospinal fluid (CSF). Taurine was four times higher than in the CSF. Perfusion with 125 mM KCl media increased particularly the taurine and phosphoethanolamine content. Nipecotic acid, a potent inhibitor of gamma-aminobutyric acid (GABA) uptake, increased GABA concentration over 60 times. In addition, it increased taurine levels by almost 10 times.