In studying the postharvest water relations of cut flowers, researchers aim to determine rates of water uptake and water loss along with changes in fresh weight. An automatic apparatus was devised for continuous monitoring of these indices. The novel apparatus consists of two balances automatically recording mass at a relatively high data acquisition rate (min−1), a personal computer, two containers, and plastic tubing. The apparatus is accurate, labor-saving, and real-time. It enabled dynamic synchronous recording of water uptake as well as fresh weight of the cut flower stem, from which precise water uptake loss rates during vase life can be accurately determined. Rates of water uptake and water loss of individual cut rose (Rosa hybrida cv. Movie Star) stems were measured using the apparatus under alternating 12-h light and dark periods. Both water uptake and water loss rates fluctuated with the light to dark shift over 120 h of observation. Stem fresh weight increased rapidly over the first 40 h of vase period and decreased gradually thereafter. Cut lily (Lilium hybrida cv. Yellow Overlord) stems showed similar trends in water uptake and water loss rate to cut rose stems. The accuracy and sensitivity of the new apparatus was validated by comparison with manual weighing using a balance at 2-h intervals under alternating 12-h light and dark periods over 108 h. The apparatus described here constitutes a suitable method for direct measurement of water uptake and fresh weight, including capturing relatively rapid water balance responses to changes in the postharvest environment.