Concentrations of LH in the serum were estimated in rats bled either once or twice during a 15-day period preceding first ovulation. In rats bled once (between 09.00 and 17.00 h) serum concentrations showed little change between 15 and 9 days before first ovulation and averaged 16 μg/l (days −15 to −9). A shift in LH level, to a mean of 31 μg/l, was seen on day −8, whereafter LH concentrations increased gradually. Basal LH values of < 10 μg/l were only found until day −4.
The finding that LH values increased with age was confirmed by data from rats bled twice with an interval of ≥ 3 days between bleedings. Furthermore, both in rats bled twice at 11.00 h and in rats bled twice at 15.00 h LH concentrations were significantly higher in the second sample.
Both morning (11.00 h) and afternoon (15.00 h) LH concentrations in rats bled once also indicated a rise in LH concentrations with age but it became apparent that only morning values showed a shift in LH concentration (from ∼ 15 to ∼42 μg/l) from day −9 to day −8. In contrast, mean afternoon values showed a gradual increase from day −15 on. From day −8 on the number of rats with LH values ≥50 μg/l increased, and from day −5 on they were more frequent at 15.00 than at 11.00 h.
Thus a clear increase in LH secretion, most likely of a pulsatile nature, was found in the female rat approaching puberty. The correlation in time and possible functional relationship with late-prepubertal follicular growth is discussed.