Observations on duration of the period of receptivity in two groups of Milking Criollo, one of Jersey and one of Brahman cows are presented. After one or two oestrus periods measured without service the cows were bred naturally in the case of Jerseys, Brahmans and one group of Milking Criollos. In the other group of Milking Criollos, artificial insemination was used. The period of receptivity was shortened after natural service by 8·7 hours in Milking Criollo and by 4·1 hours in Brahman cows. No shortening of oestrus occurred in the Jerseys and no significant shortening in artificially inseminated Milking Criollos.
Nine of the cows under observation (and seven other cows with some Zebu blood) allowed only one service by the bull.