The relationships between experimentally induced deciduoma formation, circulating estradiol (E) and progesterone (P) levels and alterations in uterine blood flow (UBF) were studied between days 4 and 15 of pseudopregnancy (PSP: day 0 = ovulation) in rats. Blood flow was measured with an electromagnetic blood flow monitor and serum analyzed for E and P levels by radioimmunoassay. Neither uterine trauma on day 4 of PSP nor the site of trauma had any direct influence on altering UBF. A dramatic increase in UBF occurred in response to stromal proliferation on days 5–6 and continued to remain above sham-operated control levels through day 9. These vascular changes correlated temporally with the day 9 peak in uterine weight resulting from deciduoma formation. Both uterine weight and vascular fluctuations in deciduoma-bearing rats were related to the cyclic changes in E/P ratios between days 5 and 8 of PSP. A decline in UBF preceded deciduoma regression between days 9 and 15 of PSP. These results suggest that an increase in UBF is causally associated with the formation and maintenance of deciduoma in the PSP rat and that deciduoma regression may result from a subsequent decline in UBF rates. Both factors may be directly dependent on fluctuating E/P ratios.