When a gland is stimulated to increase its secretion by the substance it produces, a(an) __________ system is operating?

This looks like a test question from a course book, so it would be difficult for anyone who didn't have the book to answer the question unless they had specific knowledge. You may find it better to look at your course material again or try a science forum. In the meantime you could have a look at some general information on glands which may give some help with the question.