The invention relates to a viscosimetric sensor for monitoring an analyte level in a patient or in body fluids ex vivo. In particular, the invention relates to a sensor for monitoring analytes, such as glucose levels in a patient. The sensor comprises at least two hermetically closed chambers (2a, 2b), including an activation chamber and a measuring chamber, containing a sensitive fluid (3), the chambers communicating with each other through at least one microchannel (5), and a semi-permeable barrier (4) allowing the analyte to enter or leave the chambers. Each said chamber comprises at least one flexible membrane (6, 7) configured for antagonistic variations of the volume of the chambers.