The master gland that regulates other glands and many body activities is the pituitary gland.

Here's why:

* Hormone Production: The pituitary gland produces hormones that control the activity of other endocrine glands, such as the thyroid, adrenal glands, and ovaries/testes.

* Body Functions: It also releases hormones that regulate growth, metabolism, blood pressure, and reproduction.

* "Master" Control: The pituitary gland itself is controlled by the hypothalamus, a region in the brain. This makes the pituitary a key part of the complex feedback loop that regulates the body's internal environment.