electron microscopic demonstration of hyaluronidase sensible proteoglycans at the presynaptic area in mouse cerebellar cortex by application of electron cytochemical techniques to cerebellar tissue the presence of proteoglycans was demonstrated at the axoplasmic matrix of mossy fiber endings blocks of glutaraldehyde g fixed mouse cerebellum were processed according to the following procedures a some pieces of tissue were postfixed in osmium tetroxide dehydrated by ethanol and embedded in araldite b other pieces were sectioned to 30 mum thick and then immersed in alcian blue solution ph 27 followed by osmium tetroxide fixation dehydrated and embedded in araldite gaboul procedure c parallel slices of b previous to alcian blue immersion were washed and incubated in either methanolhcl neuraminidase ribonuclease or testicular hyaluronidase with their respective controls d other blocks of g fixed tissue without any other treatment and fixation were dehydrated and embedded in araldite ultrathin sections of a b and c were doubly stained with uranyl acetate and lead citrate while ultrathin sections of d were stained with the osmium coordination compound osdmeda the electron microscopic study revealed at the presynaptic axoplasm of mossy fiber rosettes the presence of a gaboul and osdmeda positive electron dense material surrounding synaptic vesicles and continuous with presynaptic dense projections this material which coincides with cytonet distribution was resistant to neuraminidase and ribonuclease and sensible to hyaluronidase and carboxymethylation these findings permit us to conclude that the axoplasmic material of mossy fiber endings is constituted by proteoglycans in which hyaluronic acid and chondroitin 4andor 6sulphate are present the probable importance of these proteoglycans in synaptic mechanisms is also discussed