Blood levels of luteinizing hormone (LH) were measured, using a radioimmunoassay, during the oestrous cycle and during the postpartum period in 14 Brangus and 5 Friesland cows. Except during pre-ovulatory surges, basal blood levels of LH ranged between 0,2 and 5,0 ng LH/ml serum. These basal levels of LH varied more between animals than either within animals or between breeds. During the pre-ovulatory surge, LH levels increased to between 8,0 and 60,0 ng LH/ml serum. In’ 16 of the 21 cases when ovulation was associated with oestrus, the LH surge was detected on the days of oestrus. In 4 instances, the LH surge was detected 1,2 or 3 days after oestrus and in one instance, 2 days before oestrus. A high incidence of ovulation without oestrus was noted. No consistent changes were detected in levels of prolactin in the serum of two of these cows during the oestrous cycle.