Total entry, irreversible loss and recycling rates of glucose were measured in four non-pregnant female Shetland ponies before and after a 24 h fast by the continuous intravenous infusion of a mixture of [U-14C]glucose and [2-3H]glucose. 2. The post-fasting fall in the concentration of glucose and the rise in the concentration of ketones in plasma were not significant. 3. After fasting the total entry rate fell from 1.44 +/- 0.11 (n4) to 1.19 +/- 0.12 mg/min per kg body-weight (P less than 0.01) and irreversible loss fell from 1.36 +/- 0.10 (n4) to 1.05 +/- 0.10 mg/min per kg body-weight (P less than 0.02). Recycling rate did not change significantly. 4. In one pony the intravenous infusion of glucose at rates of 0.52, 1.0 or 1.64 mg/min per kg body-weight had no appreciable effect on endogenous glucose production whereas in a second pony, similarly fed, total entry was depressed by 43% and irreversible loss by 54%. 5. Irreversible loss of glucose in the horse is similar to that of many other species.
