Individuals with type 1 diabetes (T1DM) are encouraged to reduce pre-exercise rapid-acting insulin ~2 hours prior to exercise (De Feo et al., 2006). However, employing this strategy can result in pre-exercise hyperglycaemia (Rabasa-Lhoret et al., 2001; West et al., 2010) predisposing to longer-term cardiovascular complications. There is little research examining the effects of administering a reduced insulin dose at different times before exercise. Seven subjects with T1DM (31 ± 2 years; BMI 26 ± 0.3 kg/m2; HbA1c 8.3 ± 0.1 %) attended the laboratory on four occasions (after giving informed consent), each time consuming 75 g of isomaltulose and administering a 75% reduced rapid-acting insulin dose (aspart or lispro; 2.1 ± 0.2 U) , in a randomised fashion. T1DM then remained at rest for 120 (120min), 90 (90min), 60 (60min) or 30 (30min) min before completing 45 min of running at 71 ± 1%VO2peak. Serum insulin (SI; Invitron; UK) and blood glucose concentrations (BG; GEM3000, Instrumentation Laboratories, UK) were measured before and 3 hours after exercise. Data (mean±SEM) were analysed using repeated measures ANOVA. Fasting SI concentrations were similar between conditions. Also, immediate pre-exercise SI were not different between the conditions (30min 127 ± 11, 60min 139 ± 9, 90min 141 ± 6, 120min 139 ± 7 pmol.l-1, P>0.05). However, SI increased with exercise under all conditions, with the increase under 120min being greater than 60min and 30min (120min Δ83 ± 7 vs. 60min Δ49 ± 9, 30min Δ48 ± 3 pmol.l-1, P<0.05). Immediately post-exercise, SI was lower under 30min compared to 120min and 90min (30min 127 ± 11 vs. 90min 222 ± 15, 120min 222 ± 11 pmol.l-1, P<0.05). Concentrations were similar at 5 minutes post-exercise and declined comparably over the remainder of the post-exercise period (P>0.05). Fasting BG were similar between conditions. Pre-exercise BG concentrations were similar (30min +2.8 ± 0.2, 60min +3.9 ± 0.2, 90min +4.3 ± 0.2 and 120min +4.1 ± 0.6 mmol.l-1). BG dropped more with exercise under 120min compared to 30min (120min -6.4 ± 0.3 vs. 30min -4.6 ± 0.3 mmol.l-1, P<0.05). In the 60 min post-exercise period 120min BGauc was lower than 30min (120min 5.9 ± 0.5 vs. 30min 7.8 ± 0.5 mmol.l-1.hr-1, P<0.05). The greatest incidence of hypoglycaemia (BG ≤3.5 mmol.l-1) following exercise occurred under 120min (120min, n=5; 90min, n=2; 60min, n=1 and 30min n=0). Administration of a reduced rapid-acting insulin dose 2 hours before exercise increased post-exercise serum insulin and resulted in lower blood glucose and increased occurrence of hypoglycaemia following exercise.