This study investigated the high hormone (HH) phase of both natural and oral contraceptive pill (OCP) menstrual cycles to determine if pre-exercise ingestion of a concentrated sodium beverage would increase plasma volume (PV), reduce physiological strain and aid endurance of females cycling in warm conditions. Thirteen trained cyclists ( VO_2peak 52 mL·kg^-1·min^-1 (SD 2); 26 y (SD 6), 60.8 kg (SD 5)) on triphasic-OCPs (OCP, n=6) or not (NAT, n= 7) completed this double-blind, crossover experiment. Cyclists ingested a concentrated sodium (HighNa⁺: 164 mmol Na⁺·L^-1) or low-sodium (LowNa⁺: 10 mmol Na⁺·L^-1) beverage (10 ml·kg^-1) before cycling to exhaustion at 70% VO_2max in warm conditions (32°C, 50% RH, Va 4.5 m·s-1). Beverage (~628 mL) was ingested in 7 portions across 60 min beginning 105 min before exercise. Trials were separated by 1-2 menstrual cycles. HighNa⁺ increased PV (calculated from Hct and [Hb]) before exercise, whereas LowNa⁺ didn’t (4.4 (SD 1.1) vs. -1.9 % (SD 1.3), 95% CI 5.20, 6.92, P < 0.0001), with greater time to exhaustion (98.8 (SD 25.6) vs. 78.7 min (SD 24.6), 95% CI: 13.3, 26.8, P < 0.0001). Core temperature rose more quickly with LowNa⁺ (1.6°C·h^-1 (SD 0.2)) than HighNa⁺ (1.2°C ·h^-1 (SD 0.2); P = 0.04). Plasma [AVP], [Na⁺] and osmolality, and urine volume, [Na⁺] and osmolality decreased with sodium loading (P < 0.05) independent of pill usage. Pre-exercise ingestion of a concentrated sodium beverage increased PV, reduced thermoregulatory strain, increased exercise capacity for women in HH of NAT and OCP menstrual cycles, in warm conditions.