A quadrant diagram for estimating fluid and sodium replacement rates. This prediction uses heart rates obtained during steady-state cycling in the heat (36°C, 50% relative humidity). Data were collected from six, fully-hydrated and unacclimatised females. Regressions show 95% confidence intervals. Commencing from position 1, enter the quadrant to successively approximate the oxygen consumption (position 2), core temperature (position 3) and sweat rate (position 4) that would obtain for the measured heart rate. Predictions of sodium (position 5) and fluid replacement rates (position 6) to sustain body-fluid and electrolyte homeostasis can then be approximated.