Increasing widespread use of St. John's Wort (SJW, Hypericum perforatum) has led to concerns about its use in pregnant women. Behavioral and physiological alterations resulting from developmental treatment were investigated in Sprague-Dawley rats exposed to diets containing 0, 180, 900, 1800 or 4500ppm SJW beginning on gestational day 3 and ending at offspring weaning on postnatal day (PND) 21. These dietary doses span 1-25 times the recommended human dose. Post-weaning behavioral assessments of male and female offspring included: open field activity, acoustic startle, performance of complex and Morris water mazes, and activity in an elevated plus-maze. There were no SJW effects on maternal weight gain or duration of gestation; offspring body weights were similar to controls from PND 2 through PND 56 after which, some treated groups weighed significantly less than the controls. There were no SJW-related behavioral alterations on any measure. Whole and regional brain weights of offspring at adulthood indicated no significant effects of SJW. These results indicate that there are few neurobehavioral alterations resulting from developmental SJW treatment in rats.