Neuropeptide Y-containing nerve fibers have previously been demonstrated to innervate the mammalian ovary. These nerve fibers innervate primarily the vasculature. In this study we have developed a method for in vivo measurement of the ovary blood flow rate by means of the 133Xe method. Using this technique we measured the ovary blood flow rate and investigated the dose-response relationship between close intraarterial-injected NPY and the ovary blood flow rate. A monoexponential washout curve for 133Xe was found for the whole washout process, ensuring that the blood flow rate at any time could be calculated from the curve. We found a mean blood flow rate in the nonpregnant rabbit ovary at 43.6 +/- 4.4 ml.(100 g)-1.min-1 (mean +/- SEM). Injection of NPY (20, 200, 2000 pM) in the aorta close to a. ovarica resulted in a dose-dependent decrease in the ovarian blood flow rate with a maximum reduction to 40.7 +/- 6.3% (mean +/- SEM) of the control blood flow rate. These findings make it likely that receptors able to interact with NPY are present in the vasculature of the rabbit ovary.