Non-lymphoid cells in rat lymph nodes are described as seen by combined light and electron microscopy of normal adult, congenitally athymic, germ-free, irradiated, or newborn rats. The cells are divided into stromal and non-stromal. The latter category consists of a variety of morphologically distinct cell types with characteristic distribution patterns. The presence of paracortical interdigitating cells in lymph nodes of germ-free rats, athymic rats, and newborn euthymic and athymic rats, refutes the ideas that interdigitating cells differentiate from macrophages under immune stimulation or T cell influences. Follicular dendritic cells are more clearly visualized and appear to be polynucleated after emptying the follicles of lymphoid cells by irradiation. Follicular dendritic cells and tingible body macrophages are found in conventionally raised euthymic and athymic rats, but not in germ-free rats. The interrelationships of these and other types of non-lymphoid cells are discussed.