The vascular source of GMH in VLBW infants is speculated to be either venous or arterial. Alkaline phosphatase (AP) staining of arteriolar endothelium makes it possible to differentiate these vessels in VLBW infants' brains and thus locate the GMH precisely. We have recently reported this technique in VLBW neonates (Pediatric Research 35:424-30,1994). We used this method to identify the vascular location of GMH in VLBW babies. Coronal whole brain slices in 6 VLBW neonates (age 23-31 weeks gestation) were embedded in celloidin, rendering the microvasculature visible by AP staining. 100μ-thick celloidin sections were examined using light microscopy and 500μ-thick sections using x-ray microscopy. Arteries, arterioles, and capillaries, but not veins and venules stained with AP. Large periventricular channels in the germinal matrix stained as veins according to AP criteria. Arterioles connected with these veins after passing through 4- to 6μ staining capillaries. No arteriovenous shunts or rete were found. GMH was found in 4 patients. In all 4 the extravascular blood was seen adjacent to veins in every case, and in a single instance next to an arteriole (which may be a secondary phenomenon). Conclusions: 1.) Neonatal vascular endothelium contained AP from 23 weeks gestation and the location of AP in arteries, arterioles and capillaries is confirmed. 2) Rupture of periventricular veins is a prominent feature in GMH in VLBW infants. Supported by NIH NS 20618 and the March of Dimes #6-FY95-223.