The present study examined the relationships between spatial learning and circulating levels of plasma estradiol and testosterone in adult male and female meadow vole, Microtus pennsylvanicus. Meadow voles are induced ovulators and most females that are housed with females or in isolation are in constant diestrus, whereas most females that are housed with males are in constant behavioral estrus. In this study sexually mature, adult male and female meadow voles housed with either females (constant diestrus) or males (constant behavioral estrus) were required to learn the spatial position of a hidden, submerged platform in the Morris water maze. Individual voles were tested using two blocks of four trials twice a day for 3 days for a total of six blocks. Task retention was examined with a probe trial 1 day after the last acquisition trial. Females were divided into two groups based on the median level of plasma estradiol [High Estradiol (15.79 +/- 1.20 pg/ml) and Low Estradiol (6.22 +/- 2.79 pg/ml) Females]. Males were similarly divided on the basis of median plasma testosterone levels [High (2.53 +/- 0.96 ng/ml) and Low Testosterone (0.45 +/- 0.08 ng/ml) Males]. High Estradiol females exhibited significantly longer latencies to reach the hidden platform, indicating poorer acquisition, than did either males (P = 0.025) or Low Estradiol females (for Blocks, 2, 3, 4, and 6, P = 0.037). Male superiority in spatial learning performance was evident only when High Estradiol females were compared to males. There were no significant performance differences between High and Low Testosterone males. There were also no group differences in retention, with all voles displaying significant retention of the spatial task. There was, however, a significant correlation between plasma estradiol levels in females and retention, with higher estradiol levels being associated with poorer retention. These results suggest that levels of estradiol in adult female meadow voles are significantly related to spatial learning, with low levels of estradiol being associated with better spatial learning. There was no evidence that levels of testosterone were related to spatial performance in adult male meadow voles. The results suggest that estradiol may have activational effects on spatial learning in the adult meadow vole and that sex differences in spatial learning are evident only when High Estradiol females are compared to adult males.