Assessment is one of the key elements in the development of the curriculum since it forms a cycle of interdependence with the other curricular areas (content, methods, etc.). This work presents the changes detected in the conceptions about assessment in science that a sample of 311 prospective primary teachers had before and after a training course presented from a constructivist perspective. This course was designed to follow the principle of isomorphism, with the educator striving 'to teach how to teach and to do so by teaching'. Information was acquired through a pre-test and post-test, 12-item, Likert-scale questionnaire. The items formed two groups of six: one typical of a traditional assessment and the other of a formative assessment. Each group consisted of two items for each category studied: the purpose of assessment, its content and the methods used. The responses were subjected to different analyses (principal components, descriptive, inferential and effect size). The results showed that initially, the trainees identify themselves as belonging to an inquiry-based model of working in class. They also showed a certain insecurity about questions that are very characteristic of traditional assessment. After the course, significant differences were found in their agreement with statements typical of a formative assessment and a reduction of their uncertainties concerning traditional assessment.