'Summer cells' of the adrenal gland in Rana catesbeiana were investigated by means of fluorescence microscopy and electron microscopy. Orthophthaldialdehyde treatment of freeze-dried tissues revealed that the summer cells possess histamine in the cytoplasmic granules. Based on the 'Faglu' technique, the summer cells were found to be free of monoamines. Ultrastructural examination showed that each summer cell was invariably surrounded by lipid cells and was segregated from other tissue components including neighbouring summer cells. Administration of compound 48/80 (a histamine liberator) evoked a rapid release of cytoplasmic granules. The envelope of the lipid cells became detached from the summer cells in the course of degranulation. Relationship between the summer cells and the mast cell family is discussed.