Yesterday, President Reagan made the last of 10 scheduled weekly Saturday afternoon talks to the nation. An hour after he finished, the seven radio networks that carried his talk also broadcast the Democratic Party's reply, delivered by Sen. Howard M. Metzenbaum of Ohio.

Strange as it may seem, this unremarkable sequence represents a significant breakthrough in the cause of public policy debate. These talks were, from everything I can learn, the longest sustained exchange of views between spokesmen of the opposing parties, under near-equal conditions, in recent political history.

Nor is this the only good news on this front. Twice this year, the major television networks have afforded the opposition party the opportunity for quick replies to televised addresses by the president, under terms that provided access to audiences of equal or nearly equal size.

One came after the president's State of the Union address, when CBS and NBC followed immediately with a half-hour film prepared by the Democratic Party, and ABC aired it with only a half-hour delay. The second came after Reagan's April 29 speech on the budget, which all three networks followed immediately with the response by Rep. Richard Bolling of Missouri, the Democrats' spokesman.

Congratulations are in order for both the broadcast executives and the Democratic Party officials who pushed very hard for what has been achieved.

Senate Minority Leader Robert Byrd, Speaker Thomas P. O'Neill, Democratic National Chairman Charles Manatt and their top public relations aides became both aggressive and persistent in seeking broadcast access, when they saw last year how effectively Reagan was using the airwaves to build support for his legislative program in Congress.

In one instance--confirmed by both sides--a turndown by a network news chief of the Democratic request for reply time was followed by a call to the network president from O'Neill and then by a call to the chairman of the parent company's board from Byrd. On the third try, the Democrats got what they wanted.

The result has been that on these occasions, and on the past 10 Saturdays, listeners have heard two sides of the case, not just one. The quick replies have also meant a better dialogue between the parties in the next morning's newspaper stories.

Broadcast executives from the three networks with whom I discussed this history all said they were simply following their traditional policy of "fairness," in according expression to contrasting views when the president is discussing controversial subjects.

For understandable reasons, they were all eager to maintain that there has been no change of policy. ABC News executive David Burke said, in a comment typical of many others, "If the Democrats think they have established a precedent that every time the president speaks, they will reply immediately, they're in fantasyland."

But if the policy has not changed, the practice seems different. In recent times, under presidents of both parties, the right of reply, the date, the time, the format and--in some cases --even the choice of the opposition spokesmen seemed to depend not on the wishes of the party leadership but on the whim of the broadcasters. In those days, as an O'Neill aide remarked, an opposition-party congressional leader was apt to find himself part of a smorgasbord of responses, broadcast two days after the presidential address, and given no more status than any of the interest-group spokesmen invited by the network to comment.

This year's pattern represents a big improvement over that. The parties, the political process and the public are well served by what has been happening.