The authors have used calcium ion (Ca++) sensitive field effect transistor (ISFET) probes to monitor in vivo changes in Ca++ activity in the venous blood of dogs in response to infusions of sodium citrate or calcium chloride. During eight experiments, Ca++ activity changes were monitored in response to sodium citrate infusions simulating infusion of 1, 2, or 4 units of citrate preserved blood per 6 min. The ISFET probe recorded decreases in Ca++ activity during all infusions. Periodic serum samples were analyzed off-line using an in vitro ion selective electrode instrument. Analysis of variance showed no statistically significant differences between off-line sample and ISFET Ca++ activities (p greater than 0.05). A second experiment tested the use of the Ca++ ISFET as a control sensor for maintaining a desired Ca++ level. These studies provided a range of Ca++ activity values for comparison of off-line and ISFET values. Infusion rates of sodium citrate or calcium chloride needed to maintain venous Ca++ activity at 1.0 mEq/L or 3.0 mEq/L for periods of 30 min ranged from 440 to less than 1 mg/min during maintenance periods. Observation of hemodynamics during the calcium ion control maneuvers suggests that calcium ion may be used to improve cardiovascular performance during critical care situations that involve cardiovascular instability.