Blood samples were collected once daily throughout the estrous cycle (and at more frequent intervals from the time of vaginal opening) from guinea pigs fitted with indwelling atrial catheters. Progesterone, luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in the plasma were measured by radioimmunoassay. Daily sampling showed that mean plasma progesterone levels were elevated between cycle Days 4 and 12 while mean plasma FSH levels were elevated between Days 3 and 11, though no significant variations were observed in LH levels except for a fall between Days 8 and 11 and a subsequent rise to Day 13. When samples were collected at 4-h intervals from the time of vaginal opening, peaks of plasma LH and progesterone were seen during, or immediately following the end of, the dark period of the day on which an estrous smear was collected. The peaks lasted for 1 sample in the case of LH and for 1 to 2 samples in the case of progesterone. More frequent sampling (2-h) indicated that the rise of progesterone followed the rise of LH. In no animal was there evidence of a rise in plasma FSH that could be correlated with the peaks of the other two hormones measured. The effect of sampling, which was found to affect the length of the estrous cycle, and, in particular, the interval between vaginal opening and ovulation, is discussed.
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Blatchley et al. (1976) studied this question.