Animal models have made numerous advances over the last century. This type of research has made a difference in how we look at psychological problems like depression. This article is a review of the literature on animal models of depression. The question of progress made will be explored. The effects of serotonin on many problems have been studied. This article will examine stress, learning, memory, brain-derived neurotrophic factor, ovarian hormone withdrawal, and the effects of certain medications. Let's look at stress and serotonin first. A stress study by Grippo, Sullivan, Damjanoska, and colleagues (2004) shows that chronic mild stress causes behavioral and physiological changes and can modify the function of serotonin receptors in rats. Sixty-four Sprague-Dawley rats were used in this study. Half were men and half were women. The following stimuli were used as stressors which include: Continuous light for 2 12 hour periods Tilt of 40 degrees along the vertical axis of the cage for a 6 hour period Paired housing for a 16 hour period and a 4 hours Wet sheets with 300ml of water poured onto the bedding for a period of 16 hours Water deprivation for 16 hours Empty water bottle after 16 hours of water deprivation for 1 hour Strobe light with 300 flashes per minute for a period of 6 hours and a 4 hour period White noise at approximately 90 dB for a 4 hour period of continuous noise and a 3 hour period with intermittent random noise All stressors were administered over a period of one week and presented in a random for another 3 weeks for a total of 4 week trial. All rats were injected with a receptor agonist and decapitated 15 minutes after trunk blood injection. The 4-week study resulted in rats showing signs of depression and satisfied adrenocorticotropic hormone (ACTH) responses to the receptor agonist. The trials also elicited a lack of pleasure in both male and female rats. Another stress-related study was conducted by Gregus, Wintink, David, and Lalynchik (2005). This study looked at corticosterone injections and stress and how these relate to depression. Sixty naïve male Long-Evans rats were used. Rats were randomly assigned to 4 groups and underwent corticosterone (CORT) injections, vehicle injections, repeated restraint stress, and repeated handling. These treatments were administered for 21 days. The CORT group and vehicle group were given injections at random times during the light and dark cycles.