The ACSM recommends that for exercise exceeding one hour, individuals can benefit from consuming a 4-8% carbohydrate solution at a rate of 600-1200 ml/hr. The purpose of this study was to evaluate these guidelines in the context of an indoor group 'aerobics' session. Methods: Nineteen individuals (6 male!1l female) participated in three 90 minute group'aerobic'sessions (-80% HRmax) during which they consumed no !luid, flavoured water (0 carbohydrate) or an 8% CHO solution (Powerade®), at a rate of900 ml/hr (total= 1350 ml). Results: The subjects incurred greater fluid losses (p=O.OO) when they consumed water (1.00±0.28l) or Powerade® (1.09±0.l2l) as compared to no !luid (0.75±O.J1l), mostly as a consequence of greater (p=O.OO) urine production. Sweat production was lower in the no fluid trial (0.62±0.21l) than the Powerade® trial (0.77±0.241, p=0.02)' but no different from the water trial (0.69±0.25l, p=0.25). There was a trend forthe Powerade® to cause a smaller rise in heart rate throughoutthe session (p=0.08) but the increase in the rating of perceived exertion was similar for all trials (p=0.49). No trial recorded significant or dangerous rises in core temperature. The trials in which fluid was consumed were no more provocative of gastrointestinal symptoms including burping, reflux, nausea and bloating. There was a trend (p=0.07) towards fewer symptoms oi'stitch'when no fluid was consumed. Conclusions: Consuming water or a carbohydrate solution according to the ACSM gUidelines did not appear to provide clear benefits or disadvantages during a 90 minute group 'aerobics' session performed in a temperate indoor environment