Mate choice is a critical driving force in evolution yet the mechanisms underlying it remain poorly understood. Ultimately, mate selection requires the perception of specific cues that indicate the characteristics of prospective mates and a decision based on those cues. While a variety of studies have investigated the nature of sensory signals emitted by animals, very little is known about the downstream processes that underlie mate assessment. I hypothesize that mate relevant sensory cues are translated by the brain into a common set of variables that reflects the value of the mate emitting them and upon which a decision to mate or not is based. To pursue this hypothesis, my goal is to identify brain structures specifically involved in mate assessment. As potential candidates, I believe that Luteinizing Hormone Releasing Hormone neurons in the hypothalamus can play a critical role in this behaviour, since these neurons control sexual behaviour and also receive information about both the internal state of the animal and the external environment. To pursue this project, my first aim is to establish a behavioural paradigm in which mate choice is stable and reproducible, using mice as model system. To do this, the value of two prospective mates will be controlled by manipulating differentially their genotype at Major Histocompatibility Complex or Major Urinary Proteins loci, which are known to influence mate preference in mice. The second aim is to identify brain regions involved in mate preference and selection. To do so, I will analyze the activation of Immediate Early Genes (IEG) that are reliable markers of neuronal activity, in the female brain during the mate choice assay. Fos immunoreactivity will first allow me to do an unbiased screen of the entire mouse brain. Second, by using cat-FISH (Fluorescent In Situ Hybridization) and exploring the temporal dynamics of two different IEG, Homer 1a and Arc, I will study the representation of mates with different values.