It has been demonstrated that neonatal administration of monosodium glutamine (MSG) results in clearly defined lesions of the hypothalamic arcuate nucleus. The present study showed that neuroendocrine function changed significantly in adulthood when boby rats were injected with MSG (4 mg/g BW, ip) 2 and 4 days after their birth. The serum LH, FSH, TSH and GH and serum and urine testosterone (TS)levels and pituitary cAMP content were lower in MSG treated rats than those of intact rats, but the serum PRL level increased significantly and the testicular cAMP content didn't change. Fourty eight hours after head irradiation with 10 Gy X-rays in the male rats treated with MSG, the serum LH, FSH, TSH and GH and serum and urine TS levels tended to decrease, while the serum PRL level tended to increase and the pituitary and testicular cAMP contents didn't change. The results suggest that the functional irregulation of neuroendocrine system in MSG treated rats with extensive lesions of hypothalamic arcuate nucleus were not so significant as those of intact rats in response to irradiation.