This study presents an experimental investigation of two different types of solar systems used for heating agricultural greenhouses. The first system employs solar flat plate collectors, installed outside a polyethylene covered greenhouse. The second system, based on the selective absorption of solar energy by a heat transfer fluid (blue of methylene), employs polyethylene alveolar transparent plan collectors, installed inside a glass covered greenhouse. Experiments on both systems were simultaneously conducted, over several days. A comparative study of the performance of both systems was carried on the experimental results achieved in the plastic greenhouse heated by the first system, and on the simulated results obtained for the same greenhouse, but heated by the second system. (C) 2003 Elsevier Ltd. All rights reserved.