Posture has no effect ( Ylikorkala et al., 1973), nor does strenuous physical exercise ( Lindberg and Nilsson, 1973a Pavlou et al., 1973). The factors responsible for the variation are unknown. Thus a single sample taken from an individual will not be representative of all samples taken from the individual, and the diagnostic significance of hPL levels is increased if serial levels are examined. However, the variation over a 24-hour period is greater than that which can be attributed to the assay itself ( Pavlou et al., 1972). Maternal levels of hPL show no nyctohemeral rhythm ( Beck et al., 1965 Samaan et al., 1966 Pavlou et al., 1972 Lindberg and Nilsson, 1973a). This is much greater than that of any other protein hormone, an interesting observation in the light of the dispute as to whether hPL has any function. The rate of synthesis of hPL has been estimated at 1–12 g/day ( Beck et al., 1965 Kaplan et al., 1968). Each subject has her own characteristic half-life and this can be attributed to differences in the mode of delivery and the physiological state of the patient. The differences in half-lives between individual subjects cannot be explained by observational error alone ( Pavlou et al., 1972). As with other biological molecules the disappearance is biphasic with an initial rapid fall (yielding the half-life stated above) followed by a period of less rapid decrease. The half-life of hPL in the mother, as estimated by the rate of disappearance after delivery of the placenta, is 10–20 minutes (Spellacy et al., 1966 Pavlou et al., 1972). Chard, in Current Topics in Experimental Endocrinology, 1983 X Metabolism and Clearance

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