Epididymal sperm flushed from the caudae of mature rabbits with Tris-saline buffer containing EDTA are motile. The endogenous rate of respiration is stimulated by glucose or pyruvate. There is no further stimulation of the respiration rate by added Ca+2, and this rate is sensitive to inhibition by oligomycin which inhibits mitochondrial ATP synthesis and slows respiration, but which does not inhibit mitochondrial Ca+2 uptake with attendant increase in respiration rate. The sperm cell membrane is not permeable to Ca+2, as shown by the observation that addition of the ionophore A23187, which makes cell membranes selectively permeable to Ca+2, results in a marked stimulation of respiration by added Ca+2. The stimulated rate is insensitive to inhibition by oligomycin, showing that the Ca+2 has direct access to the mitochondria which take up this cation in an energy-linked reaction. The presence of 45 mM HCO-3 in the medium stimulates the rate with pyruvate, but not with glucose, and does not affect the response to added Ca+2. Cyclic AMP and dibutyl cyclic AMP, with or without added Ca+2, also do not stimulate respiration with glucose or pyruvate as substrate. Mature rabbit sperm from the cauda epididymis are apparently intrinsically motile, needing only freedom from restraint and oxygen; a trigger in the form of Ca+2 is not needed.