Abstract Synthetic. LRF (IV 150 μg dose) elicited a prompt increase in circulating LH during all phases of the cycle studied. The maximal response was found during preovulatory phase and the minimal response was seen during the early follicular phase. A change in LRF responsiveness with greater and more sustained LH release from the early to the late follicular phase was observed. The response during luteal phase was significantly greater than responses in both the early and the late follicular phases. A concomitant but a much smaller response in the rise of serum FSH levels was observed. These data indicate that the sensitivity of pituitary gonadotrophs to LRF is preferentially increased for LH release during the late follicular phase of the cycle and that a maximal pituitary responsiveness to LRF appears to occur at the midcycle.