Fluorescence histochemical and electron microscopic techniques were used to investigate small intensely fluorescent (SIF) cells in the ninth and tenth paravertebral sympathetic ganglia of bullfrogs. Fluorescence histochemistry revealed that clusters of SIF cells are sparsely distributed in the ninth and tenth ganglia; the clusters were usually located in the vicinity of blood vessels. Fluorescent processes were not observed emanating from SIF cells. The clusters stained positively for the chromaffin reaction indicating that SIF cells are chromaffin cells. Ultrastructurally, the SIF-chromaffin cells in the sympathetic ganglia appeared virtually identical to the chromaffin cells in the adrenal gland; this includes two cell types that appear morphologically the same as the epinephrine and norepinephrine containing chromaffin cells in the adrenal gland. Efferent synapses from the SIF-chromaffin cells in sympathetic ganglia to sympathetic neurons were not observed. The SIF-chromaffin cells in bullfrog sympathetic ganglia did not have the morphological characteristics of interneurons; it is suggested that they may function as extra-adrenal chromaffin tissue. BIBLIOGRAPHIC REFERENCE: Weight, F.F, and Weitsen, H.A.: Identification of small intensely fluorescent (SIF) cells as chromaffin cells in bullfrog sympathetic ganglia. Brain Res. 128: 213-226, 1977.