An in vivo study in rhesus monkeys demonstrated that a single injection of Δ9-THC during the midluteal phase of the menstrual cycle decreased serum progesterone levels, an effect which was reversed byThis assumptionwas consistent with previous findings from anin vivo study on ovariectomized rhesus monkeys, whichdemonstrated that a single injection of Δ9-THC decreasedserum levels of both LH and FSH the administration of hCG