We have realized that our Biology undergraduate students learn biological concepts as established truths without awareness of the body of experimental evidence supporting the emerging models as usually presented in handbooks and texts in general. Therefore, we have implemented a laboratory practice in our course of Physiology and Biophysics, aimed to introduce the students in the way the scientific models and theories are built, through the measurement of Na+ transport in frog skin. Transepithelial Na+ transport was assessed in the frog skin, with measurements of short circuit currents. The mucosal Na+ and serosal K+ concentrations were modified and the effects were recorded. These effects were reversible. Addition of a drug that blocks epithelial Na+ channels (amiloride) to the mucosal side solution abolished the short circuit current. Sodium fluxes were calculated, and the results were adjusted to Michaelis-Menten kinetics. The impact of the proposed practice on the students is discussed.