We have studied cultured rat hippocampal neurons using electron microscopic procedures based on the accumulation of DAMP (dinitroanilino-aminomethyl-dipropylamine) to identify acidified locales, fluorescence procedures to provide information about the pHs within certain endocytic compartments, and the uptake of horseradish peroxidase to evaluate effects of altering pH on membrane cycling in the axonal varicosities. We find that the endocytic compartments related to the lysosome-endosome system in the cell bodies and dendrites of these neurons maintain pHs in the range of about 5 to 6.5. This is the range that would be expected for structures participating in lysosomal digestion and for such functions as the endosomal dissociation of ligands from receptors. We also find that, as judged by uptake of horseradish peroxidase, exposure of the preparations to weak bases that neutralize intracellular compartments does not abolish the endocytic labeling of synaptic vesicles in the axonal varicosities. This suggests that passage through a markedly acidified compartment or stage is not obligatory for the endocytic phase of the cycling of synaptic vesicles.