['The purpose of this study was to determine whether gestagen treatment enhances the effects of beta2-mimetics in hormone-induced preterm delivery in pregnant rats in vivo.', 'Preterm birth was induced with a combination of mifepristone and prostaglandin E2 on day 19 of pregnancy. The animals were treated with salmeterol or gestagens (progesterone or 17alpha-hydroxyprogesterone) or their combination. The treatments were launched on different days (15-18) of pregnancy. The efficacy of treatment was determined in terms of the delivery time counted from the mifepristone injection.', 'Salmeterol treatment delayed premature labor by 2.4 hours, whereas the delay because of the gestagen-salmeterol combinations was more than 5 hours. Progesterone had no effect on the delivery time.'] Does gestagen treatment enhance the tocolytic effect of salmeterol in hormone-induced preterm labor in the rat in vivo?