A previous study has shown bidirectional saturable transport of LHRH across the blood-brain barrier. Here, the effects of the steroids progesterone and beta-estradiol and the pituitary glycoproteins luteinizing hormone (LH) and follicle stimulating hormone (FSH) on the bidirectional transport rate were determined. No statistically significant difference in brain to blood transport of 125I-LHRH was found in mice given ICV progesterone (1 and 100 pmol/mouse), beta-estradiol (1 and 100 pmol/mouse), FSH (10 and 1000 pmol/mouse) or LH (100 and 1000 pmol/mouse). Blood to brain transport of 125I-LHRH, tested in rats with a carotid artery perfusion method, was not affected by inclusion of progesterone (100 nmol/ml), beta-estradiol (100 nmol/ml), LH (2 and 10 nmol/ml), or FSH (10 nmol/ml) in the perfusate. We conclude, therefore, that unlike its release from the hypothalamus, the exchange of LHRH between the CNS and blood is unlikely to be influenced by reproductive hormones.