The purpose of this study is to develop and examine the psychometric properties of the Online Course Overload Indicator (OCOI) and the Student Mental Fatigue Survey (SMFS). The OCOI was designed to measure students' perceptions of cognitive overload in online courses. The SMFS was used to assess students' perceptions of mental fatigue while taking online courses. An exploratory factor analysis was conducted on a sample of 378 undergraduate students from various institutions offering online courses across the United States. Results of a factor and reliability analyses confirmed that the instruments are valid and reliable measures of students' perceived mental fatigue and overload from online course elements. The analysis supported the model that students' perceptions of overload in online courses consist of four constructs--information relevance, information overload, course design, and facilitation--in addition to the one-factor structure of the SMFS, which consists of the student mental fatigue construct.