Pregnant mice were treated with naloxone via subcutaneous implants, from about 5 days prior to parturition. At birth entire litters were cross-fostered so that groups of offspring were exposed to naloxone treated mothers; before birth, after birth to weaning, from about 5 days prior to birth to weaning, or not exposed to naloxone. When tested on a hot-plate at 50 days of age, females either prenatally treated or treated pre- and postnatally showed hyperalgesia to heat. For males, this effect was not evident. This sex difference may have been induced by the cross-fostering procedure.