A multicenter, open comparison of the effects of three doses of 5 cc/kg Exosurf(two 2.5 cc/kg aliquots) 12+_ 1 hours apart vs a minimum of three and a maximum of six doses of 5 cc/kg Exosurf Neonatal (each dose divided into two 2.5 cc/kg aliquots) administered intratracheally over a four day interval to extremely premature infants (birth weight _> 500 and <_ 750 grams).