Four-day cycling rats were kept in a room with the light on from 0500-1900 h. Plasma LH concentrations in blood withdrawn through atrial cannulas at hourly intervals from 1400-2000 h on proestrus were very similar to serum LH concentrations in blood collected from uncannulated rats by decapitation at these times. Additional cannulated rats were bled (0.1 ml) at 5 min intervals from 1400-1600, 1600-1800, or 1800-2000 h. After each bleeding, 0.1 ml of heparinized saline was injected through the cannula. Following a rise (rate unknown) to detectable levels of about 200 ng/ml, plasma LH displayed a rapid linear increase to 1538 +/- 118 ng/ml, starting between 1445 and 1650 h and lasting 20 to 50 min. Over the next 110 min plasma LH at first rose erratically by about another 200 ng/ml and then fluctuated around an apparent plateu. It then dropped rapidly but the declines were commonly interrupted by one or more rapid increases in plasma LH. A generalized pattern of the proestrous LH surge has been constructed from the data.