Self-injurious behavior (SIB) and stereotyped behavior (SB) are major challenges for professionals in the field of mental retardation. From animal experiments it has become obvious that these behavioral disturbances are not purposeless but may emerge secondary to restrictive environment and may serve de-arousing objectives. In mentally retarded subjects, several hypotheses have been formulated concerning the pathogenesis of SIB, particularly about the involvement of serotonin and beta-endorphin, which are supported by beneficial treatment effects of the opiate antagonist naltrexone and serotonin modulating compounds, respectively. The present study was designed to investigate basal levels of stress-hormonal and serotonergic parameters as well as plasma levels of amino-acids and the beta-carboline norharman in a group of 64 mentally retarded subjects with SB and/or SIB. Allocation to three different groups comprising 17 retarded controls, 26 subjects with mainly SIB and 21 subjects with mainly SB, was originally performed using the scores on the factors Irritability, Stereotypic Behaviour and Hyperactivity of the Aberrant Behavioral Checklist. Because of the overlapping nature of the behavioral parameters, subjects were subsequently divided into three maximally contrasting groups, viz. predominantly SIB, predominantly SB and retarded controls, each comprising 11 subjects. With respect to beta-endorphin, no differences were found either between both the original and maximally contrasting groups or in comparison to nonretarded controls. As compared to retarded controls, a tendency to lower values for total cortisol and cortisol binding globulin appeared to be present in the SIB group, whereas in the SB group a tendency toward higher levels of the major serotonin metabolite 5-HIAA was found. In the contrasting SB group, a trend toward decreased total cortisol level was observed as compared to the retarded control group. In addition, significantly lower values for norharman and tryptophan were demonstrated in the total group of mentally retarded subjects as compared to non-retarded controls. The results of the present study, yielding co-existent disturbances in stress-hormonal and monoaminergic mechanisms as well as in the metabolism of norharman, are in line with the hypothesis that mentally retarded subjects are at risk for the development of stress-related behavioral disorders such as SIB and SB.