The Antimalaria drug, chloroquine is an amphiphilic cationic compound and might therefore be suspected to interfere with foetal lung maturity as previously observed with other amphilphilic cationic drugs. Single doses of chloroquine phosphate (40mg/kg b. wt) was administered to pregnant rats on days 20 and 21 of gestation (term = 22 days). Morphometric analysis revealed a decrease in volume density (Vv) of parenchyma, saccular space and average saccular volume. Concurrent administration of single doses hydrocortisone on day 21 increased the Vv of the Parenchyma saccules and average saccular volume. This result suggests that the retardation of foetal lung maturity induced by chloroquine could be reversed by a concurrent administration of hydrocortisone. Questions concerning the mechanism by which these effects are produced must remain open.