It appears that ISFET probes can reliably and continuously monitor IF K+ in vivo for intervals of at least several hours. The consistently observed increase in IF K+ in response to hemorrhage, a phenomenon invisible systemically, suggests that such probes may provide clinically valuable information regarding perfusion related events at the cellular level during onset of and resuscitation from hypoperfusion states. Precise correlation of ISFET signal to specific cellular dysfunction awaits investigation in which muscle cell membrane potential, muscle surface pH, and postexperiment cellular histology are studied concurrently.