1. Regeneration of involuted thymuses of old rats follows double suprarenalectomy in this species with great constancy.

2. Partial thymectomy alone does not induce compensatory secondary hyperplasia in involuting glands.

3. Regeneration of the thymus following suprarenalectomy would appear to be completed in most rats in about 2 weeks; and it probably begins in the medulla.

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