The integrity of the immature diaphragm may critically influence the susceptibility to respiratory failure after preterm birth. We aimed to investigate the effect of LPS administered at single or multiple doses at different time points during gestation on the function of preterm diaphragm. This study was approved by the Animal Ethics Committee of The University of Western Australia. Pregnant Merino ewes received ultrasound guided intra amniotic injections of LPS (10 mg) or saline. LPS exposures consisted of single injections at either 2, 7 or 21 day (d) or multiple injections (7+14+21 ds) before operative delivery (pentobarbitone;150 mg/kg I.V) at 121 d gestational age. Preterm lambs were killed with pentobarbitone (150 mg/kg I.V) and the right hemi-diaphragm was dissected. One diaphragm strip was frozen in liquid nitrogen for skinned fibre force analysis. Another strip was mounted in an in vitro muscle test system and contractile parameters and fatigue resistance were assessed (1). For skinned fibre analysis, muscle strips (control and 21 d LPS) were processed as described by Ottenheijm., 2010 (2). Small bundles of fibre were attached to the force transducer and activated in solutions of different pCa and pSr (4.1-8.0). Single 21d and multiple LPS exposures during gestation significantly reduced P0 by ~40 %, relative to saline control (Table 1). The Pt decreased by ~30 %, and was accompanied by a significant increase in ½ RT and TTP. 21d LPS exposure did not alter either the pCa50 or Fmax compared to controls. The severity of contractile dysfunction was greater when the initial exposure occurred 21 d prior to delivery and multiple LPS exposures did not cause additional impairment in contractile function. The skinned fibre results suggest that the muscle weakness (at 21 day exposure) is mediated by altered calcium handling rather than myofilament dysfunction.