It was previously shown that 0.18 µg actinomycin-D (Act-D) in bilateral implants into the preoptic area (POA) significantly inhibited lordosis behavior in the ovariectomized estrogen-primed rat [Quadagno et al., 1971]. This experiment was repeated in order to correlate the suppression of steroid-induced sexual behavior with certain ultrastructural features of cells in the POA. Cells in the POA of animals receiving Act-D showed profound structural changes in nucleolar morphology which are typical of cells altered by Act-D treatment. It is suggested that Act-D is producing a specific effect, i.e. blocking the DNA-dependent synthesis of messenger RNA, rather than a nonspecific effect due to cytotoxicity.