The process of implantation of the blastocyst in the endometrium is considered analogous to a “proinflammatory” response, and a possibility exists that any compound showing good anti-inflammatory activity may also possess a significant anti-implantation potential. Leaf extract from the shrub Vitex negundo Linn has previously been studied for its analgesic and anti-inflammatory activities. Hence, anti-implantation activity of the methanolic extract of leaves of Vitex negundo Linn was investigated. Pregnant female mice were dosed with the extract (500 mg/kg body weight) from Days 1 to 6 of pregnancy. No implantation sites were observed in treated animals when they were surgically opened on Day 15 of pregnancy. Biophysical alterations were observed in the endometrium in treated animals, especially on Day 5 (4:40 a.m.), the day of implantation. A sharp increase in superoxide anion radicals that was seen in the endometrium from control animals was altered in treated animals. Thus, the physiological alterations induced by extract of Vitex negundo Linn during the process of implantation may serve as a good lead for further research on natural contraceptive targets.