Propranolol concentrations will be measured on dried blood spots during the first 3 days of treatment and at the steady state. Cardiovascular and respiratory parameters will be continuously monitored. Blood samplings checking metabolic, renal and liver functions will be performed periodically, as well as cardiac function, in order to verify the treatment safety. Serial ophthalmological evaluations will be planned to monitor the efficacy of the treatment, the ROP progression and the possible complications.