FERTILITY-CONTROL IN LABORATORY RATS AND MICE AFTER FEEDING WITH THE ANTIGESTAGEN RU486

The intermittent use of an antigestagen could prove to be a very effec tive way of controlling the fertility of rats and mice in the wild. Th is concept was tested by giving paraffin wax blocks containing cereal grains and the antigestagen Mifepristone, RU486 (150 mg kg(-1) block) to male and female laboratory rats and mice in a series of free-choice feeding experiments. There was no significant difference in the consu mption of blocks with or without RU486, showing that it was completely palatable to rats and mice, and no aversion developed following refee ding. The average consumption of RU486 by rats was 11 mg kg(-1) day(-1 ); mice consumed 37 mg kg(-1) day(-1). All the females showed persiste nt oestrous vaginal smears throughout the treatment. When male and fem ale rats and mice were given continuous access to treated paraffin blo cks for 30 days, no conceptions occurred. At the end of this time, the re was a significant increase in ovarian weight in the treated rats an d mice, but no difference in testicular weight. Treated blocks were gi ven to rats for 3 days every 21 days for a total of 115 days. Four dea d litters were produced following the first antigestagen treatment on day 21, but no more litters were produced and no treated rats were pre gnant when autopsied on day 115. Mice were initially treated for 3 day s every 21 days, but some animals continued to produce live young on t his schedule. The treatment period was therefore reduced to 3 days eve ry 18 days and no more litters were produced and none of the treated f emales was pregnant at autopsy. The antigestagen RU486 shows considera ble promise as a chemosterilant for the control of fertility in female rats and mice. Intermittent administration every 18 days (mice) or 21 days (rats) in free-choice feeding trials completely inhibited reprod uction.