PERIODICITY is characteristic of the sexual behaviour of lower mammals. In animals with oestrous patterns, the female will accept the male only when she is “in heat”, a condition relating temporally to ovulation and the presence of certain gonadal hormones. Primates are different; apes and most monkeys, like humans, have menstrual cycles, and mate at all times during the cycle. Laboratory studies of several species of primates, however, show a periodicity of sexual behaviour within the menstrual rhythm with increased sexual behaviour occurring at mid-cycle when ovulation is believed to occur1.