It is well known the problem presented by laboratory activities when we attempt to promote meaningful learning. One possible solution to this problem is to design and test open experimental activities which would approximate student’s work to scientist’s work. This paper proposes the design of two open laboratory guides, constructed in the light of a constructivist model, with suggestions taken from experience and from the existing literature, emphasizing the need of qualitative designs stressing conceptual construction and allowing students to play with their own ideas during the experiment. A comparative study between traditional and open laboratory guides was carried out. Research findings show that the open guides favor conceptualization, since a better conceptual understanding was facilitated by the non-normative character of these guides. Key-words: experimental activites; open guides; traditional guides.