1. Efflux of radiocalcium from both microsomal and mitochondrial fractions of flounder ileum was potentiated in the presence of caffeine. In the presence of ACh an increase in radiocaleium efflux was only observed in microsomal fractions. 2. Sections of spontaneously active flounder ileum maintained at 10°C became quiescent when placed in calcium-free media. The tissue then exhibited several single contractions in response to small doses of either caffeine or acetylcholine (ACh). 3. After several additions of either caffeine or ACh tissues failed to respond. 4. The number of doses of ACh or caffeine required before tissues failed to contract was strongly temperature dependent. 5. In the presence of ACh ileal cells were seen to depolarize whereas the presence of caffeine lead to hyperpolarization. 6. It is concluded that caffeine and ACh access different elements of smooth muscle cellular calcium pools and the ability to access these pools is temperature dependent.