Abstract
In seeking a method for the routine observation of copulatory behaviour in rats we compared the use of a standard rectangular Makrolon cage, commonly used in toxicology studies, with a Plexiglas cylinder that provided 2·5 times more floor area. The cylinder resulted in a much higher incidence of copulation, either once or for multiple series, within 30 min. This was not affected by whether oestrus was natural or induced. Using the cylinder and 4 h observation periods, we found that the dark phase of a 12–12 h inverse light cycle resulted in many more copulations than occurred during the light phase of a natural cycle. The incidence increased from the first to the second to the last 4 h of the dark phase. We found that placing a virgin receptive female and a naïve male together in a Plexiglas cylinder for 1 h towards the end of the dark phase is a useful tool in reproductive toxicology studies in which it is important to know the precise time of insemination.