Calcium accumulation in muscle strips isolated from rat diaphragm reached steady state after 90 min with intracellular to extracellular ratio (IN/OUT) of 3.0. Calcium uptake by rat diaphragm is a saturable and carrier mediated process that is significantly enhanced in the presence of aminophylline in a dose-dependent pattern that produced maximal effect at 18.75 micrograms/ml. A Lineweaver-Burk plot shows Jmax = 25 microM/hr per g dry wt and Kt = 3.19 mM for the control, and Jmax = 26 microM/hr per g dry wt and Kt = 2.27 mM in the presence of 18.75 micrograms/ml aminophylline. Aminophylline had no effect on cell volume and water of the diaphragmatic cells.