Contraceptive steroid treatment to female rat alters the concentration of acetylcholine and gamma-aminobutyric acid, though not uniformly in most of the brain areas, i.e. in cerebellum, medulla oblongata-pons, hypothalamus, striatum-hippocampus, midbrain-thalamus-subthalamus, and cortex. Both neutrotransmitters vary conjointly in the hypothalamus and striatum-hippocampus after combined steroid treatment. This may indicate a concerted effect on the modulation of gonadotrophin release.