1. Mating of mice does not occur with equal frequency during the first 4 nights after pairing. Fewer than the expected number mate on the 1st, 2nd and 4th nights, but mating on the 3rd night greatly exceeds expectation.
2. This effect is not dependent on age or parity, and is little altered by pairing the mice at 11 p.m. when oestrous behaviour is fully established.
3. Mating occurs more frequently on the 1st night after pairing if the male has been enclosed in a small basket within the female's cage for 2 days previously. Also, the incidence of mating is altered by placing the females in a cage recently contaminated by males.
4. The oestrous cycle of the mouse is apparently modified by the presence of a male or his excreta, and an exteroceptive stimulus functioning through one of the chemical senses is probably involved.