The transfer of tritiated water across the isolated, artificially perfused guinea pig placenta was the subject of 21 experiments. The observed relationship between the flow rates and the relative transfer of water cannot be explained by a concurrent or pool-flow system. If the direction of the fetal flow is reversed, the rate of transfer is lowered. It may be concluded, that the decrease is a result of a change from a nonideal countercurrent flow system to a nonideal concurrent system. This conclusion, however, holds only if all other parameters that determine the exchange as well remain unaffected. In the range of flows investigated, the transfer of water is flow limited.