The reproducibility of LH and testosterone responses after Gn-RH treatment in young post-pubertal bulls was tested. Ten bulls were injected i.m. (250 .mu.g) twice with an interval of 4 wk. Blood samples were collected every 15 min for 2 1/2 h and then on the 2nd occasion, every hour. LH increases very rapidly and reaches an initial peak by 30 min after Gn-RH injection. This increase over the basal level was 25 to 30-fold. After a plateau of high levels lasting 40 to 50 min, a 2nd peak of LH concentration occurs. LH then decreases and returns to basal levels by 6 to 7 h after treatment. The increase of plasma testosterone was more prolonged and of smaller magnitude than that of LH, 3-4-fold. There was a large and significant variation between bulls, but with LH, a given individual responded in a consistent manner on the 2 separate occasions. LH response to Gn-RH treatment might be useful and should be tested as of diagnostic value for clinical or breeding purposes. The testosterone response does not seem to be as valuable and reliable as that of LH.