Kisspeptin is the product of the Kiss1 gene that regulates GnRH secretion. In sheep, Kiss1 mRNA-expressing cells are found in the preoptic area (POA) and arcuate nucleus (ARC), and expression is up-regulated in the caudal ARC during the periovulatory period. We hypothesized that kisspeptin neurons in the ARC are activated by estradiol-17beta prior to the preovulatory LH surge. Ovariectomized ewes were treated as follows: 1) estradiol-17beta implants (sc 2 wk) to cause tonic negative feedback; 2) vehicle (no estrogen negative or positive feedback); or 3) positive feedback/GnRH surge-inducing injection of estradiol-17beta (50 microg iv). For groups 2 and 3, brains were collected 1 h after treatment and kisspeptin/Fos immunoreactivity was examined. In the caudal and mid-ARC, the percentage of kisspeptin cells that were Fos immunoreactive increased after acute estradiol treatment (group 3) over that seen in the other two groups. Kisspeptin/Fos colocalization was also quantified in ewes during the luteal and late-follicular phase of the estrous cycle, showing a trend toward an increase in colocalization in the late-follicular phase. Kisspeptin/Fos colocalization was similar in the POA across groups in both experiments. Analysis of Kiss1 mRNA by in situ hybridization revealed an increase in expression during the late-follicular phase in the caudal ARC and POA. These data suggest kisspeptin neurons located in the caudal extent of the ARC are involved in generating the positive feedback preovulatory GnRH/LH surge in the ewe, but there may also be a role for Kiss1-expressing cells in the POA.