Female hamsters were ovariectomized and one week later stereotaxically infused with 20 micrograms cycloheximide (20 micrograms/microliter, 0.6 microliter/min) or 1 microliter saline bilaterally into one of several brain sites. Thirty min after the infusion, the animals were injected with 5 micrograms estradiol benzoate (EB) then 44 hr later with 500 micrograms progesterone and tested with a male for receptivity at hour 48. One week later the animals were retested with EB and progesterone but without any intracranial infusion to investigate the possibility of permanent lesion effects. The greatest inhibition of receptivity occurred in females which received cycloheximide in the ventromedial hypothalamus (VMH) or lateral to the VMH. Saline infusions into the VMH had no inhibitory effects. Inhibition was rarely seen after infusion into the preoptic area, anterior hypothalamus, dorsomedial hypothalamus, cortico-medial amygdala, or septal area. Moderate inhibition was found after infusion of cycloheximide into the third ventricle. The inhibition following cycloheximide infusion was not permanent or irreversible. On the retest one week later the animals were sexually receptive. These results suggest that protein synthesis within the VMH is an essential part of the initial action of estrogen for the induction of sexual receptivity in hamsters.