The purpose of this study was to investigate, in Wistar rats, the effects of a treatment with H. perforatum administered prenatally and during breastfeeding (from 2 weeks before mating to 21 days after delivery). Two doses of the extract were chosen, 100 mg/kg per day, which, based on surface area, is comparable to the dose administered to humans, and 1000 mg/kg per day. A microscopical analysis of livers, kidneys, hearts, lungs, brains and small bowels was performed. Severe damage was observed in the livers and kidneys of animals euthanised postnatally on days 0 and 21. The lesions were more severe with the higher dose and in animals that were breastfed for 21 days; however, important renal and hepatic damage was evident also with the dose of 100 mg/kg per day. In addition, similar serious hepatic and renal lesions were evident also in animals that were exposed to hypericum only during breastfeeding. In particular, focal hepatic damage, with vacuolisation, lobular fibrosis and disorganisation of hepatic arrays was evident; in the kidney, a reduction in glomerular size, disappearance of Bowman’s space and hyaline tubular degeneration were observed.