Behavior of rhesus monkeys during 28-day cycles of estrogen treatment.

Sixteen ovariectomized rhesus monkeys were paired, each with two males, during thirty-two 28-day periods during which they received one or more of the following schedules of daily injections: (a) different mixtures of estradiol benzoate, progesterone, and testosterone propionate (artificial menstrual cycles), (b) the doses of estradiol benzoate given in artificial cycles but without the other hormones (cyclic estradiol regimens), and (c) appropriate control treatments (625 one-hour behavioral tests). Hormone doses were adjusted to give plasma levels in the physiological range (335 blood samples). Measures of male sexual activity during artificial cycles and during cyclic estradiol regimens showed significant cyclic changes, but these changes were indistinguishable from each other. In four females studied in an operant conditioning paradigm, there was a midcycle dip in the times taken to obtain access to males during the cyclic estradiol regimens. The data indicated that the estrogen state of the female plays a preponderant role in determining the cyclic changes in the behavioral interactions of the pair independently of the effects of other ovarian hormones.