The acquisition of conceptual knowledge in scientific domains is among the central aims of school instruction because this semantic declarative knowledge helps individuals make interferences and explain complex phenomena. Recent research shows that naïve concepts acquired during childhood persist in long-term memory long after learning the scientifically correct concepts in school. In this study, we investigated the effects of stress on the retrieval of these conceptual representations. To this end, 40 healthy men were randomly assigned to either psychosocial stress or a control condition and evaluated, as quickly and accurately as possible, statements that were compatible with scientific concepts or incompatible with those concepts. Some of these statements were true and some were false. Incompatible statements in this case are statements which are in line with adults' scientific concepts, but not with children's naïve theories. In contrast, compatible statements are in line with both. Stress induction was successful as evidenced by increases in blood pressure and cortisol concentrations in the stress group compared to the control group. Responses were delayed and less accurate for incompatible compared to compatible statements. Psychosocial stress had no main effect on retrieval, but abolished reaction time differences on false- vs. true-incompatible statements. This effect was mirrored in correlations between individuals' cortisol increases and reaction times. These results suggest that stress, as embodied by increases in cortisol concentrations, interferes with the retrieval of conceptual knowledge. They help to better understand conceptual knowledge retrieval in real-life situations such as examinations or problem solving in the workplace.