1. The effect of the different phases of the menstrual cycle on skeletal muscle strength, contractile properties and fatiguability was investigated in ten young, healthy females. Results were compared with a similar group on the combined (non-phasic) oral contraceptive pill (OC). Cycle phases were divided into the early and mid-follicular, mid-cycle (ovulatory) and mid- and late luteal. Cycle phases were estimated from the first day of the menstrual bleed. 2. Subjects were studied weekly through two complete cycles. Measurements included quadriceps and handgrip maximum voluntary isometric force and the relaxation times, force-frequency relationship and fatigue index of the quadriceps during percutaneous stimulation at a range of frequencies from 1 to 100 Hz. 3. In the women not taking the OC there was a significant increase of about 11% in quadriceps and handgrip strength at mid-cycle compared with both the follicular and luteal phases. Accompanying the increases in strength there was a significant slowing of relaxation and increase in fatiguability at mid-cycle. No changes in any parameter were found in the women taking the OC. 4. The changes in muscle function at mid-cycle may be due to the increase in oestrogen that occurs prior to ovulation.