There is perhaps nothing more worthy of careful consideration as a human problem than the suggestive results of the behavior of our experimental rats as modified by diet. They have varied from the most highly satisfactory type for the promotion of growth, fertility and the preservation of vigor, to the opposite extreme when little or no growth could take place and physical deterioration developed apace leading to early death. The well nourished rat is large and muscular and has a glossy coat, bright eyes and a quiet disposition. When placed in new surroundings it shows a strong desire to explore its environment. As soon as this is done it settles down to rest, and at no time, so long as it remains on familiar ground, does it manifest much activity. Its nervous system is stable and its threshold of stimulus relatively high. It is not apprehensive of danger and does not manifest much curiosity as to its surroundings except when placed upon new territory. Such a rat may be handled several times a week by lifting it by the tail by means of tongs, which causes some discomfort, yet it will fail to remember the experience or develop as a result fear or resentment at being handled in the same manner or with the unprotected hand. A cage containing half a dozen such animals can be opened without causing them to rise from a resting posture, and one can reach in with a bare hand and take them by the tails, pulling them about so as to collect the entire group in one hand, and can withdraw them as a single handful and demonstrate them to a visitor with their heads down for several seconds without causing them any alarm. They do not resent being handled, even being held suspended with their heads down, although this causes them some discomfort, and they do not attempt to bite.