The evaluation was prompted by the controversy about informed consent as a requirement for using prisoner subjects in experiments. Based on actual experience with the problems of conducting research in prisons, the study emphasizes that volunteers need better information about key aspects of an experiment in order to grant truly informed consent. The experiment compared the effect of lithium carbonate versus placebo in modifying the frequency of threatening or violent behavior or reducing anger in the delinquents. All subjects signed a consent form, and the 3-year, double-bind study attempted to provide 'free and informed consent.' This latter effort was evaluated with an unannounced questionnaire administered to subjects after they complete the experiment. At that time, approximately two-thirds had sufficient information for an informed consent, but only one-third was well informed about all key aspects of the experiment and one-third was insufficiently informed to give an informed consent. Institution-based or study-based coercion was minimal in the experiment. Evaluation of the questionnaire and experiment at the study institution showed that an experiment with human subjects should be designed to include an ongoing evaluation of informed consent, and active attempts should be made to avoid or minimize coercive inducements. Experiments with significant risk, which require a long duration or large sample size relative to the institution's population, should probably not be performed on prisoner subjects. The experimenter should be independent of the penal institution's power structure. Presenting and explaining a consent form to volunteers on one occasion is probably an informed consent. Several tables and 17 references are provided. (Author abstract modified)