Abstract
Serum progesterone and LH levels were measured in the same serum samples at various stages of the estrous cycle in 7 cyclic heifers. Progesterone values were determined by gas liquid chromatography, and LH levels were measured by a solid phase radioimmunoassay, using an antiserum to bovine LH. The radioimmunoassay was specific for LH as judged by several criteria and serial dilutions of standards and unknowns gave parallel curves. Various tests indicated that immunoreactive LH as measured by the radioimmunoassay was related to biological activity. Most animals had basal levels of serum LH of 2–4 ng/ml. A preovulatory LH peak (7–50 ng/ml) was observed in every heifer and LH levels declined rapidly following this peak. Every heifer showed a small luteal phase LH peak and 5 of the 7 heifers showed another LH peak 4–7 days before ovulation. These fluctuations in basal LH levels may be correlated with growth and atresia of anovulatory follicles. Progesterone levels were very low following estrus (>100 ng/100 m...
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