The present thesis examines evidence for the self-relaxation assumption of Personality-Systems-Interactions (PSI) theory, i.e. the involvement of the self-system in the intuitive downregulation of negative affect. Chapter 1 introduces PSI a self-regulation theory that explains personality and behavior according to the dynamic interplay of neuropsychological systems. Chapter 2 provides an evaluation of the Implicit Positive and Negative Affect Test (IPANAT) that indirectly assesses affect via judgments about the phonetic resemblance between artificial pseudo-words and mood adjectives. The test shows remarkable reliability for each scale as well as good factorial, construct and criterion-based validity. Chapter 3 presents an experiment that investigates the extent to which self-activation (experimental presentation of self-referential terms such as my bed, my body, etc.) after stress exposure affects negative and positive affect as measured by both the IPANAT and an explicit affect scale. As compared to neutral terms (the bed, the body, etc.) in the control group, self-referential terms led to a decrease in negative affect and an increase in positive affect according to the IPANAT but not according to the explicit measure. The results are discussed with respect to the intuitive nature of self-relaxation. Chapter 4 includes a study that examines the relationships between inter-individual differences in self-functioning (adult attachment security, self-determination, self-esteem) and the cortisol response to an acute stressor and awakening in the morning. Self-functioning in terms of attachment security was negatively related to the acute stress cortisol response and, along with self-determination and self-esteem, positively related to the awakening cortisol response. A hypothesized link between the hippocampus, which has been shown to be especially important for cortisol regulation and the retrieval of episodic memory, and the self-system is discussed.