Purpose: This study investigated the effects of the menstrual cycle on running economy (RE). Method: Eleven eumenorrheic female athletes (mean age: 21.18 [plus or minus] 3.65 years, height: 170.2 [plus or minus] 6.6 cm, VO[subscript 2]max: 49.25 [plus or minus] 9.15 mL[middle dot]kg[superscript -1][middle dot]min[superscript -1], and menstrual cycle: 29.8 [plus or minus] 0.98 days) were tested for anthropometric variables, physiological responses (oxygen consumption [VO[subscript 2]], blood lactate [LA], heart rate [HR], and respiratory exchange ratio [RER]) at rest and while running. The RE was measured at speeds of 75%, 85%, and 95% of the lactate threshold at 3.5 mmol[middle dot][superscript L-1] during the follicular (FP) and luteal phases (LP) of the menstrual cycle. The RE was evaluated as oxygen consumption (mL[middle dot]kg[middle dot]min[superscript -1] [O[subscript 2C_]min], mL[middle dot]kg[superscript -1][middle dot]km[superscript -1] [O[subscript 2C_]km]) and caloric unit cost (kcal[middle dot]kg[superscript -1][middle dot]km[superscript -1] [E[subscript C]]) during both phases. Results: There were no significant differences in body composition or resting physiological measurements between the LP and FP (p > 0.05). Physiological responses measured during RE tests were similar in both phases (p > 0.05). The RE measured as O[subscript 2C_]min, O[subscript 2C_]km, and E[subscript C] was significantly lower during the LP than during the FP (p < 0.05). The RE defined as O[subscript 2C_] min significantly increased with speed (p < 0.05), but RE defined as O[subscript 2C_]km and E[subscript C] was unaffected by speed increment (p > 0.05). Conclusions: The RE is better in the LP than the FP and is independent of running speed when RE is evaluated as O[subscript 2C_]km and E[subscript C]. The menstrual cycle had no effect on body composition and physiological variables measured at rest.