Substance P (SP) and luteinizing hormone releasing hormone (LH-RH) content were measured by radioimmunoassay in median eminence of female rats, SP only in rats pituitary after decapitation on the day of oestrus (E), on the first and second day of dioestrus (D1 and D2), on the day of proestrus (P) and in "constant oestrus" (AD) caused by anterior hypothalamic deafferentation and in lactation (L). During four days of oestrus cycle the highest SP and LH-RH content of median eminence was found in dioestrus. Both during cyclic oestrus and "constant oestrus", SP and LH-RH were significantly lower than on the second day of dioestrus. These results may suggest the reciprocal functional relation of both hypothalamic peptides in the mechanism of the control of gonadotropin release. The highest LH-RH concentration of the median eminence was found in the animals on the 11th to 15th day of lactation, while the SP content in the median eminence of these animals is not significantly different from those of the remaining groups except the value for dioestrus (D2) rats. The SP content in pituitary of lactating rats was significantly higher than those of rats during cyclic or "constant oestrus".