The results of several studies imply that estrogen can act upon the central nervous system via a protein synthetic step. Our objective was to determine if the intrahypothalamic (preoptic area, POA) injection of cycloheximide (CHX), an inhibitor of protein synthesis, at 17.00 h on diestrus II of the 4-day cycle altered lordotic behavior and (or) ovulation in the intact rat (sexual receptivity to males normally begins on the evening of proestrus as herein defined; ovulation occurs on estrus of the cycle). CHX-treated females were tested for lordotic behavior at 23.00 h on proestrus, then killed at 17.00 h on the following day. None of the CHX-POA rats were receptive to the males and 90% of these rats did not ovulate. Thus, CHX significantly suppressed sex behavior and ovulation in the cyclic rat.