Electrolytic lesions of the hypothalamic medial forebrain bundle (MFB) produced substantial deficits in the copulatory behavior of male rats, but no changes in testicular, seminal vesicle, ventral prostate or penis weights. Radioimmunoassay of serum from lesioned and control animals revealed a small but statistically insignificant reduction in testosterone and LH levels, and no change in FSH levels of lesioned males. In a second experiment, androgen levels were maintained in castrated males by subcutaneous implants of silastic capsules containing crystalline testosterone; this procedure failed to prevent the copulatory deficit which accompanied subsequent MFB lesions. We have concluded that the behavioral deficit produced by such lesions is not a direct result of pituitary-gonadal dysfunction.