U.S. milk production varies throughout the year, with production being highest in the spring and lowest in the fall. On average from 1982 to 1992, milk production was 7 percent higher in the second quarter compared to the fourth quarter of the year. Milk production seasonality is mainly due to two related factors. First, a greater proportion of cows freshen in the spring than during the rest of the year because of the availability and quality of pastures (Kaiser, Oltenacu, and Smith). Second, cow productivity tends to be highest in the spring, due to the quality and availability of pasture. Seasonality in production is a problem for many processors of manufactured dairy products because plants and equipment used to satisfy spring production levels may operate at less than capacity for much of the year. Seasonality also adds to the cost of federal dairy programs because the government, under the dairy price support program, acts as a seasonal balancer of supply and demand by removing excess supplies of cheese, butter, and nonfat dry milk from the market. Evening out these seasonal imbalances in milk production would reduce the costs of the price support program and lessen the costs to processors dealing with excess capacity. In the past, several federal and state milk marketing orders have employed seasonal price incentive plans to encourage more even milk production throughout the year. These seasonal price plans are designed to discourage spring production and encourage fall production. For example, the Louisville Plan, which has been used in several markets, places an assessment on the milk price in the spring, and then adds back the proceeds and interest in the fall. There are other plans as well, but most operate by making he spring milk price lower and the fall milk price higher than it would be in the absence of the program. The purpose of this article is to examine the impacts of a hypothetical seasonal priceincentive plan on milk production seasonality in t United States.' Seasonal milk production is compared with alternative levels of price differentials that are deducted from the spring milk price and added to the fall milk price. Understanding the impact of seasonal price incentive plans on cow and heifer numbers, production per cow, and milk production, requires a dynamic milk supply-response model with biological constraints. Hence, a secondary bjective is to develop a quarterly dynamic milk supply-response model that explicitly accounts for both biological constraints on dairy cattle and economic factors that influence dairy herd size. The model is similar to an annual dynamic milk supply-response model developed by Chavas and Klemme. A system of nonlinear equations for numbers of heifers and cows, and production per cow is specified according to constraints on the age structure of the cohort and farmers' seasonal adjustment decisions on culling and replacing cows. A quarterly-based supply-response model with biological constraints on dairy herd size is needed to simulate the impact of seasonal price differentials on seasonality because farmers will adjust seasonal culling and replacement decisions in response to such programs. This article differs from previous research (e.g., Hall, Oltenacu, and Milligan; Prindle and Livezey; Caine and Chin-Hwa Sun is an Associate Professor in the Institute of Fisheries Economics at National Taiwan Ocean University. Harry M. Kaiser and Olan D. Forker are Associate Professor and Professor, respectively, in the Department of Agricultural, Resource, and Managerial Economics at Cornell University. The authors thank Tim Mount and Les Myers for valuable comments on earlier drafts of this paper. Financial support from the Commodity Economics Division, Economic Research Service of the United States Department of Agriculture is gratefully acknowledged. IIdeally, it would be more appropriate to consider seasonality at a more regionally disaggregated level. Unfortunately, not all of the data required for the model are available at the regional level.