DURING investigations into the possibility of artificial insemination of indigenous Zebu and Nganda cattle with homologous semen obtained by electrical stimulation, the œstrous cycles of an experimental group of heifers have been determined, using marked vasectomized bulls. The animals have been examined for evidence of service at 0700 and 1900 hr. local time. In two months, œstrus has been detected in five animals on fifteen occasions; but marking by the bull has never occurred on less than two occasions. These were recorded as of 12 hours duration (0700–1900 hr., and vice versa). Based on this marking of the females, three heat periods lasted 12 hr., nine lasted 24 hr., two lasted 36 hr. and one lasted 48 hr. Of this total, six (40 per cent) were first recorded at 0700 hr. (commenced during darkness), and nine were first recorded at 1900 hr. (commenced during daylight).