The effectiveness of use-value assessment provided under the California Land Conservation Act in maintaining open space and in deterring the conversion of prime agricultural land to urban uses is evaluated in the San Joaquin subbasin for the period 1958-1974. Results from the prime land resource use-stock model indicate that the Act is a significant policy variable exhibiting the intended effects upon idle and agricultural land stocks, but that it is ineffective in controlling the growth of urban stocks on prime land.