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Plasma luteinizing hormone (LH) levels were measured by radioimmunoassay in serial samples taken from intact adult cockerels, gonadectomized fowl of both sexes and laying hens. By sampling at 30 and 10 min intervals, it was shown that in cockerels LH is released episodically. Each secretory episode lasted 90–120 min and was characterized by a 100–200% rise in LH levels over a period of 10–15 min followed by a more gradual, exponential decline. Mean plasma LH levels were found to be depressed in cockerels as a result of the handling associated with taking blood samples; in cases where LH levels were most depressed, an episodic pattern of secretion could not be demonstrated.

In the gonadectomized fowl, an episodic pattern of LH secretion was shown when blood samples were taken at 10 or 5 min intervals. During each secretory episode, which lasted 20–45 min, LH levels rose by 20–60%. It is suggested that the increase in mean plasma LH levels in gonadectomized fowl (30–77 ng/ml) over those in cockerels (7–16 ng/ml) is related to an increase in the frequency of the episodic release of the hormone. Episodic discharges of LH could not be demonstrated in laying hens. In these birds the mean LH levels are low (1·5–2·4 ng/ml) and consequently may be maintained by secretory episodes which are of too low an amplitude to be detectable.

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