A commercial conductivity detector was used to monitor the changes in the conductivity of the cervical mucus in Mashona and Afrikaner beef cows synchronised with cloprostenol in an attempt to predict the optimum time for artificial insemination. The recorded conductivity values, prior to, during and after oestrus were of a similar level and pattern to those found by other workers, the nadir occurring at or close to oestrus. There were no differences in the pattern or level of conductivity readings between breeds nor in those from animals that conceived and those that did not. Similarly, there were no differences between animals that displayed signs of oestrus and those that did not. Insemination to the lowest conductivity reading, after two cloprostenol injections 11 days apart, gave higher conception rates for both Mashona and Afrikaner cows (57 and 40 per cent), than fixed-time inseminations at 72 and 97 h (42 and 19 per cent), or insemination to observed oestrus after a single cloprostenol injectionfollowed by a further injection and fixed-time insemination for those not responding to the initial injection (50 and 19 per cent). Of those animals inseminated to lowest conductivity readings, 69 per cent of the Mashona and 83 per cent of the Afrikaner conceived to inseminations at the nadir, the remainder conceived to inseminations on the day after the nadir. In both breeds 30 to 40 per cent of animals showed their lowest conductivity reading on the days prior to the onset of oestrus. Insemination of these animals to their lowest reading resulted in a proportion conceiving which suggests that in some cows fertile ovulation occurs substantially earlier than the onset of behavioural oestrus. The broad temporal spread of onset of oestrus and lowest conductivity readings, 48 to 168 h after the second synchronising injection, will also mitigate against high conception rates from any combination of two fixed-tune inseminations in these breeds of cows.