What arouses an animal or human from an inactive, nonresponsive state to a condition of activity and responsiveness? What are the biological mechanisms for this change? In this book Donald W. Pfaff focuses on a reproductive behavior typical of many female animals. Sensory stimuli from the male trigger responses in a well-defined circuit of nerve cells. At the top of the circuit, certain nerve cells receive and retain sex hormones such as estrogens and progesterone. As a result, specific genes in these nerve cells are turned on at specific times, affecting in turn the rest of the neural circuit and causing a state of sexual responsiveness. According to Pfaff, the biological bases for the most primitive human drives are largely explained by mechanisms uncovered in animal brains that have not changed in their fundamental properties over millions of years of evolution. Focusing on a single instinctive behavior, in this case the sex drive, is an important step toward understanding the biological reasons for the change from unmotivated to motivated animal behavior.