delta-Aminolevulinic acid (ALA), a toxic precursor of porphyrin, when injected i.v. failed to enter most brain structures, such as the cortex, thalamus, midbrain, and others, as determined by the autoradiographic technique. However, the choroid plexus, median eminence, and a narrow strip of the periventricular zone, which usually lack a barrier function, were labeled in these experiments. These results indicate that the blood-brain barrier is virtually impermeable for ALA, although the amino acid may enter the cerebrospinal fluid via fenestrated capillaries of the circumventricular organs and tanycytes of the median eminence.