The effects of whole-body potassium depletion induced by food deprivation on plasma, erythrocyte, and middle gluteal muscle K concentrations was quantified in 16 healthy, adult horses before, during, and at the end of a 7-day period of food deprivation during which water and sodium chloride were available ad libitum. Potassium concentrations were determined by atomic absorption spectroscopy. Plasma K concentration remained constant (3.49 +/- 0.09 mM K/L of plasma; mean +/- SEM) throughout the study. Erythrocyte potassium concentration decreased from 93.10 +/- 1.94 mM K/L of erythrocytes on day 0 to 88.63 +/- 2.39 mM K/L of erythrocytes on day 2 (decrease of 4.8%; P less than 0.05) and thereafter did not change. The K concentration of the middle gluteal muscle decreased from 91.06 +/- 2.96 microM K/g of muscle (wet weight) to 79.61 +/- 2.09 microM K/g of muscle (decrease of 12.6%; P less than 0.05) on day 4 and decreased further on day 7 to 73.62 +/- 1.85 microM K/g of muscle (decrease of 19.2%; P less than 0.05). There was no correlation between the plasma and erythrocyte K concentrations (r = -0.066), the erythrocyte and middle gluteal muscle K concentrations (r = 0.167), or the plasma and middle gluteal muscle potassium concentrations (r = -0.018). The water content of the middle gluteal muscle remained constant (73.23 +/- 0.36%) throughout the study. Erythrocyte membrane potential did not change (-99.26 +/- 0.87 mV) during the study, whereas the magnitude of the membrane potential of the middle gluteal muscle decreased from -105.84 +/- 1.67 mV on day 0 to -100.93 +/- 2.10 mV on day 7 (P less than 0.05).