Plasma LH and progesterone levels were measured daily from the sixth to the fifteenth day of the oestrous cycle in Friesland cows during December (summer) of one year, and April/May (autumn), June/July (winter) and September (spring) of the following year. Five to seven cows were sampled per season, and they were subjected to a plane of nutrition designed to maintain body mass throughout the experimental period. Mean tonic LH levels during autumn (2,26 ± 0,07 ng/ml) were significantly (P ≤ 0,01) higher than those obtained during summer (1,95 ± 0,06 ng/ml) and spring (1,94 ± 0,07 ng/ml), but not significantly higher than those obtained during winter (2,15 ± 0,07 ng/ml). The mean area under the LH curve for cows sampled during autumn was significantly (P ≤ 0,05) greater than that for cows sampled during summer and spring. The season did not influence progesterone concentrations. The possible reasons for the seasonal differences in tonic LH secretion are discussed.