Subjective perceptions of mental and physical energy were measured by the use of visual analogue scales (VAS) over the course of a week in a group of normal volunteers. These variables were found to correlate, highly both with one another and with assessments of vigour and fatigue measured with a standard adjectival format. In addition, high correlations were found with a measure of positive affect. These five variables also displayed characteristic patterns of diurnal variation. Physical and mental energy, vigour and positive affect were highest in the morning, falling progressively and significantly over the day. In contrast, fatigue showed the opposite pattern. Extraversion showed positive correlations with physical and mental energy, vigour and positive affect, and negative correlations with fatigue and negative affect, most being significant (P less than 0.05). Conversely, neuroticism showed a negative correlation with the first four energy variables but was positively related to fatigue and negative affect, most correlations again being significant. Physical and mental energy, vigour and positive affect also showed a positive correlation (P less than 0.01) with a measure of happiness. It is concluded that VAS-derived ratings of physical and mental energy are reliable indicators of self-perceived vigour and fatigue. They are easy to record and show sufficient short-term stability to be used in more extensive studies both of well-being and of its physiological correlates.