The neuronal selectivity of protargol silver was determined in embedded and cultured nervous tissue by light (LM) and transmission electron microscopy (TEM). Ultrastructurally, silver (85-140 A in diameter) was found deposited in neuronal, but not glial, perikarya and cell processes. Usually, the silver grains were arranged in elongated arrays that were associated with electron-dense, strand-like structures that correspond to the 'neurofibrils' observed in LM; myelin sheaths, mitochondria, Nissl substance, Golgi bodies, synaptic vesicles, and endoplasmic reticulum could be excluded as the basis for neuronal argyrophilia. Unfortunately, the composition of these neurofibrils was obscure because of poor tissue preservation involved with proper impregnation, but they appeared to correspond to microtubules and neurofilaments. Neuronal structures containing a predominance of either organelle were impregnated with protargol. Nuclei and nucleoli of neuronal and nonneuronal cells were densely stained with silver as visualized with LM and TEM. Not every neuron was impregnated with protargol, but under conditions of improper fixation some of these neurons had an electron-dense cytoplasm which exhibited a definite affinity for silver. A discussion of protargol selectivity is also presented.