Monensin, a growth-promoting agent and coccidiostat used in veterinary medicine, was studied in terms of its effects on the development of female rats and their offspring when added to the diet. Young female rats received 100 or 300 ppm monensin mixed with powdered chow diet until adulthood, when they were mated and their offspring were evaluated for physical and neurobehavioural development. The data showed that 1) exposure to the higher monensin concentration reduced female body weight; no deaths occurred after exposure to the two monensin concentrations; 2) the offspring of the experimental group receiving 300 ppm monensin presented a weight reduction from 10 to 21 days of lactation; 3) incisor eruption was delayed only in females after exposure to the 100 ppm concentration. We conclude that exposure to monensin during development induces toxicity in female rats and has a notable adverse effect on growth with some limited effects on selective milestones of physical and functional development of the offspring during the postnatal period.