Following are excerpts from President Reagan's letter yesterday to Sens. William S. Cohen (R-Maine), Charles H. Percy (R-Ill.) and Sam Nunn (D-Ga.) on arms control and the MX missile. Most of the letter is the same as one sent to House members Wednesday. The new parts used here have to do with the idea of a weapons "build-down" about which the senators particularly asked the president.

You have suggested that certain additional initiatives could be helpful in moving us toward our goals of security and stability at reduced levels of forces. One of the most prominent of these initiatives is the idea of a "guaranteed build-down."

The principle of a mutual build-down, if formulated and implemented flexibly and negotiated within the context of our modified START strategic arms reduction talks proposal, would be a useful means to achieve the reductions that we all seek.

It would, if properly applied, reinforce our intent to cap the number of strategic ballistic missile warheads on both sides and to cause each side to reduce those levels steadily and substantially over time.

It could be implemented flexibly and with reasonable latitude for each side to balance the forces it deploys and reduces. Variable ratios as appropriate would encourage more stabilizing rather than less stabilizing systems.

It could be implemented in conjunction with an agreed floor which, when reached, would trigger the suspension of the build-down rule, subject to renegotiation.

As you have acknowledged, any build-down concept must recognize the importance of strategic modernization and the necessity of maintaining a balance during the reduction process to deal with asymmetries in U.S. and Soviet forces. It would, of course, require agreement on effective verification measures, including counting rules for all systems.

My administration is currently examining the structure of a build-down proposal which would meet these criteria and would facilitate a START agreement embodying substantial reductions in nuclear forces. I will work with you and your colleagues to develop such a proposal.