Conservative Republicans are up in arms about the refusal of Republican National Chairman Bill Brock to invest $50,000 of party funds in a cross-country congressional caravan against ratification of the Panama Canal treaties.

Brock refused a direct plea for the money from Ronald Reagan and Sen. Paul Laxalt (R-Nev.) late last week, provoking what Laxalt called an "angry" reaction from GOP conservatives.

Reagan was reported to be particularly indignant because earlier this year he had signed a fund-raising letter for the Republican National Committee, seeking contributions to "conduct a full-fledged campaign to alert citizens to the dangers Republicans see in this treaty."

Officials at the RNC said the letter had brought in $700,000. Last Friday, Franklyn Nofziger, a Reagan political aide, wrote Brock and the Republican Congressional Campaign Committee, which had also used the Reagan letter for fund-raising, telling them to "cease and desist" further mailings.

Nofziger said Reagan was "not trying to start a feud with the RNC, but it's a serious problem. This ought to be a two-way street."

Laxalt, who last year was chairman of Reagan's almost-successful campaign for the Republican presidential nomination, said the incident "reaffirms my feeling that if we are going to be effective as conservatives, it will have to be outside the RNC. It's obvious they're not sympathetic to our goals."

Brock, in a telephone interview from Florida, confirmed that "Reagan was upset" when they spoke on the phone for more than an hour last Thursday.

"I can understand that," the chairman said. "He has done an awful lot for us. But I think he understand it was just not possible for us to do this."

Brock said he received many requests from Republicans to finance independent efforts on particular issues, but said he could not "get into a position of choosing which particular activities and groups to help, that are not part of an overall program."

The January air caravan to eight cities is being organized by Laxalt and Rep. Philip M. Crane (R-Ill.) under the aegis of the Committee to Save the Panama Canal. Laxalt aides said about 20 House and Senate members are expected to participate in the effort.

Laxalt said funds will be sought from private contributors to meet the anticipated $100,000 cost of the effort.

"Obviously," he said, "there's going to be a strong feeling among conservatives around the country" about Brock's refusal to pay half the cost from party funds.

The split in GOP leadership ranks was dramatized when former President Ford on Sunday reaffirmed his support for the treaties. Senate Minority Leader Howard H. Baker Jr. (R-Tenn.) is still publicly neutral on the question.

However, the Republican National Committee voted opposition to the treaties "as proposed by President Carter" at its October meeting, and the fund-raising letter distributed by the RNC that month over Reagan's signature said ratification would be "one of the most serious mistakes" in American history.

Brock, in defending his denial of the funds, noted that the letter said contributions were needed to "defeat those who vote time and time again against a strong U.S. foreign policy." He said the money would be used to aid Republican candidates next November.

Brock also said he had no objection to financing anti-treaties speeches around the country "if they are part of the Republican leadership program in Congress. But I feel strongly we should be addressing a whole range of issues, not just the treaty question." CAPTION: Picture 1, BILL BROCK; Picture 2, RONALD REAGAN, . . . GOP chairman rejects plea from former candidate who helped raise $700,000.