In the course of a longitudinal research program conducted in Sweden, data were being collected on biological and psychological aspects of individual functioning and on environmental factors for a fairly large representative sample (approximately 1,000) of Swedish males and females between 10 and 27 years of age. Based on data from the longitudinal study, this report focuses first on relationships between males' conduct, school age, adult criminality, and physiological factors. Findings indicated that aggressiveness at school age is highly predictive of later criminality, whereas the absence of aggressive tendencies at an early age is strongly related to the absence of criminal offences at adult age. When aggressiveness at age 13 was related to adrenaline excretion at the same age, a significant negative correlation was obtained. Significant and substantial differences between criminals and non-criminals were found in mean adrenaline excretion at school age. Subsequent analysis of early aggressiveness and motor activity as predictors of adult criminality found that motor activity was a stronger factor than aggressiveness per se. (RH)