A formative assessment pretest was administered to undergraduate students at the beginning of a science course in order to find out their prior knowledge, misconceptions and learning difficulties on the topic of the human respiratory system and energy issues. Those findings could provide their instructors with the valuable information required in order to adapt their teaching methods to the students' needs. The test included open-ended questions and was administered on the first day of the course. The data obtained were analysed in relation to the students' gender, age and having attended or not attended advanced courses in biology at the high-school level. Students could have prior knowledge on a topic to be learned, which, if identified and accounted for in the teaching, could serve as a receptor for a constructivist mode of study. The results indicated that undergraduate students hold misconceptions which could obstruct the acquisition of new knowledge. They encounter learning difficulties, which, if are known to the instructors and addressed in their teaching, could facilitate students' learning. The possible use of a formative pre-assessment procedure, which could guide the instruction and learning process from the beginning of a course, is discussed.