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The kinetics of prolactin turnover were investigated in female rat adenohypophyses in vivo. The animals received an intravenous injection of [³H]leucine and groups were killed at various times thereafter. The prolactin in their adenohypophyses was separated and measured by disc electrophoresis and densitometry. The hormone band was cut from the gel columns and counted by liquid scintillation procedures. The specific activity data (c.p.m./µg) were fitted to single or double exponential functions by the method of least squares, using a computer program. The slow component of the double exponential, or the single component of the single exponential were assumed to represent secretion and their rate constants were multiplied by the hormone content of the gland to obtain an estimate of secretion rate. The assumptions involved in this method of estimating secretion rates are discussed and evaluated.

The rate constant for prolactin in oestrogen + reserpine-treated females (0·10–0·08 h^–1) was higher than that obtained in rats treated with oestrogen alone (0·05–0·06), and approximately three times that in rats treated with reserpine only (0·03). The prolactin concentration in the adenohypophysis was highest in reserpine-treated rats (8·2 µg/mg) while in glands from oestrogen-treated (4·1–6·0 µg/mg) and oestrogen + reserpine-treated animals (5·0–6·2 µg/mg) it was similar. The average prolactin secretion rates in oestrogen-treated (0·26 µg/mg/h) and reserpine-treated (0·25 µg/mg/h rats were similar, while the average rate in reserpine + oestrogen-treated rats (0·54 µg/mg/h) was twice as high. The results with reserpine-treated animals (low rate constant, high gland content), with oestrogen + reserpine-treated animals (high rate constant), and of those previously obtained by us in ectopic adenohypophysial transplants (high rate constant, low gland content) are discussed in terms of possible involvement of a prolactin stimulating factor. It is concluded that this method provides useful estimates of secretion rates as well as information on the intrapituitary turnover of prolactin.