Purpose: This study investigated the effects of the menstrual cycle on running economy (RE). Method: Eleven eumenorrheic female athletes (mean age: 21.18 +/- 3.65 years, height: 170.2 +/- 6.6 cm, VO(2)max: 49.25 +/- 9.15 mL center dot kg(-1)center dot min(-1), and menstrual cycle: 29.8 +/- 0.98 days) were tested for anthropometric variables, physiological responses (oxygen consumption [VO2], 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 center dot L-1 during the follicular (FP) and luteal phases (LP) of the menstrual cycle. The RE was evaluated as oxygen consumption (mL center dot kg center dot min(-1) [O(2C_)min], mL center dot kg(-1)center dot km(-1) [O(2C_)km]) and caloric unit cost (kcal center dot kg(-1)center dot km(-1) [E-C]) during both phases. Results: There were no significant differences in body composition or resting physiological measurements between the LP and FP (p > .05). Physiological responses measured during RE tests were similar in both phases (p > .05). The RE measured as O(2C_)min, O(2C_)km, and E-C was significantly lower during the LP than during the FP (p < .05). The RE defined as O-2C_ min significantly increased with speed (p < .05), but RE defined as O(2C_)km and E-C was unaffected by speed increment (p > .05). Conclusions: The RE is better in the LP than the FP and is independent of running speed when RE is evaluated as O(2C_)km and E-C. The menstrual cycle had no effect on body composition and physiological variables measured at rest.