Factors that limit the performance capabilities of sustained artillery operations in simulated conventional and chemical warfare environments were studied. The results show that perceptions of psychological (mental) fatigue, rather than perceptions of muscular fatigue, were primary factors affecting sustained artillery performance. Furthermore, variations in these psychological states were correlated with artillery task performance during the period. In the simulated chemical warfare environment, extreme symptom and mood changes resulted in medical casualties, combat ineffectiveness, and early termination of all testing. Significant perosnality differences existed between casualties and survivors. The majority of casualties voluntarily terminated operational duties because of intense symptoms associated with wearing the chemical protective mask and clothing system. These symptoms were manifestations of respiratory and thermal stress.