This paper develops a simple procedure to estimate the extent to which party pressure affects roll-call voting in the U.S. congress. We find strong evidence of party pressure in both the House and the Senate, in virtually all congresses over the period 1871-1995. We do not find any large, systematic differences in pressure between the House and Senate. Over the post-war period party pressure in the House occurs especially often on key procedural votes – the rule on a bill, motions to cut-off debate, and motions to recommit. Pressure appears most frequently on budget resolutions, tax policy, social security, social welfare policy, and the national debt limit, while it is relatively rare on moral and religious issues and civil rights, and entirely absent on issues such as gun control. There have been dramatic changes in the extent of party pressure on some issues, such as agriculture, public works, and nuclear energy policy.