 This study is designed to provide data about the pharmacokinetics PK, safety and tolerability of two continuous treatment regimes of tobramycin nebulized solution delivered via a soft mist nebulizer in Cystic Fibrosis CF subjects. Each treatment period will last 8 weeks. Additionally the PK of patients with a normal forced expiratory flow in 1 second FEV1 FEV180 predicted will be compared to patients with an abnormal FEV1 FEV180 predicted. 