The objective of the present study is to assess the contribution of bulk flow to the regulatory mechanism of amniotic fluid volume and its ionic concentration in the membranes surrounding the amniotic fluid. For quantitative assessment, we prepared 4 kinds of artificial amniotic fIuids (isotonic isovolumetric, hypotonic isovolumetric, isotonic hypervolumetric and hypotonic hypervolumetric ones) by replacing 70% of amniotic fluid of pregnant rabbits with water or normal Tyrode solutions. Isoosmotic saline of 0.5 ml volume containing 0.05% Censored and 15 mM/l LiCl was administered initially into amniotic sacs of all subject animals. Samples of amniotic fluid were collected in after 30 and 90 minute intervals; the concentrations of Censored, Na⁺ and Li⁺ were determined and compared. Followings are the results obtained. 1. from isovolumetric and increased Congcord group, we couldn t find significant change in Li⁺ and Na⁺ concentration in isotonic amniotic fluid. However, Na⁺ concentration increased significantly as well as a striking increase in Censored concentration in hypotonic amniotic fluid. 2. In isovoIumetric and decreased Censored group, the rate of [Li⁺] decrement and the rate of [Na⁺] increment were much higher in hypotonic amniotic fluid than in isotonic. 3. In hypervolumetric and increased Censored group, the rate of Na⁺ efflux increased proportionately with the increment of Censored concentration up to 0.98, which was higher than the rate of Li⁺ efflux in isotonic amniotic fluid. However, the increment of Na⁺ concentration was rather related with the initial Na⁺ concentration in hypotonic amniotic fluid, showing inverse relationship. Li⁺ concentration increased only when there was a marked increase in Censored concentration and approached near a maximum value or 1. 4. For hypervolumetric and decreased Censored group, the observations were identical to isovolumetric and decreased Censored group. From these results the following conclusions could be made: 1) There is no net movement of water or monovalent cations across the membranes surrounding amniotic fIuid in isotonic isovolumetric condition. In contrast, there is a net efflux of amniotic fluid by osmotic bulk flow, resulting in elevation of Na⁺ concentration in hypotonic isovolumetric condition. 2) In hypervolumetric conditions, there is a massive efflux of amniotic fluid or solvent drag through the surrounding membranes by fiItrative bulk flow, where the rate of Na⁺ efflux has a linear relationship with that of water efflux. This is assumed to be carried out through enlarged and newly opened intercellular spaces resulting from increased intraamniotic pressure. 3) Once increasing intraamniotic pressure reaches a point allowing Li⁺ to pass through during osmotic bulk flow in hypotonic amniotic fIuid, Na⁺ influx seems to occur by diffusion simultaneously or immediately thereafter, too.