Treasury Secretary James A. Baker III yesterday said that lower oil prices, the cheaper dollar and lower interest rates have combined to provide the best economic outlook for Third World nations since the early 1970s.

But Baker warned that improved growth among the debtor nations, based on internal economic changes, will be "essential to any resolution of the debt problem." Without substantive changes, Baker said, "no amount of money" will solve the problem of overhanging debt, which is expected to hit $1 trillion by the end of 1986.

In two speeches to the International Monetary Fund's Interim Committee -- which began a two-day meeting here yesterday on a range of global economic issues -- Baker stressed the debt problem related to international monetary changes, which have preoccupied the major nations for much of 1985.

He called for more cooperation between the World Bank and International Monetary Fund to foster economic growth in the Third World. He added that the IMF has not always "given adequate priority" to structural policy changes critical to growth in debtor nations and to reversing capital flight.

Such structural changes, Baker said, include more public enterprise into the private sector, growth-oriented tax policy, trade liberalization and development of more efficient domestic equity and capital markets. "We look for further adaptation of fund lending policies" to take such changes into consideration, Baker said.

In his brief remarks on monetary issues, Baker said the present exchange rate system "has both strengths and weaknesses." With recent cooperative efforts -- an allusion to meetings of the Group of Five major industrial nations -- he noted that exchange rates are now "more in line with fundamentals." Nonetheless, he supported further studies of how to improve the system.

The Interim Committee is expected to endorse Baker's position on monetary changes in a communique today.

A European official at yesterday's session said it was clear that Baker "was moving away from the idea of a new international monetary conference."

Baker's emphasis on debt, rather than on exchange rates, reflected the mood of most key finance ministers and central bankers here that the sharp drop in the dollar -- as well as an inability to agree on a framework for changes -- has removed the pressure for immediate action.

Baker was instructed by President Reagan in his State of the Union message to study whether a new global conference should be convened to encourage more stability in exchange rates. He was given until the end of the year to come up with an answer.

Baker and many Europeans prefer to keep discussions of exchange-rate relationships within smaller groups. Thus, the communique Tuesday by the Group of 10 leading industrial nations "stressed the need" of keeping discussions on exchange rates and surveillance of individual economies "within the framework of the IMF."

During the past few days, interest has grown in bolstering the IMF's surveillance authority to discuss domestic economic policies with major nations when they get out of line with each other, using a series of "objective indicators" as a test.

The IMF Interim Committee is expected to endorse improved surveillance procedures in its communique today. It will consider a specific recommendation Baker made yesterday -- that the IMF managing director give increased publicity to his assessment of a given country's economic performance.

Some officials said yesterday that the idea of a global monetary conference could be revived by the heads of state at the Tokyo economic summit in May.

In his remarks on debt issues, Baker said that the initiative he had proposed last October in Seoul at the IMF/World Bank annual meeting was the correct approach, and is being widely supported by commercial banks and the major lending institutions. He said that it should not be referred to as a "Baker plan" that promises a quick fix, but viewed "as a common agreement on how to proceed in helping debtor countries overcome their financial debt problem."

In Seoul, Baker proposed the kinds of economic adjustments in debtor nations that he mentioned yesterday, as well as increased lending by commercial banks and multilateral development agents, such as the World Bank.

Baker yesterday said the slide in oil prices would cut oil-import costs for Third World importers by $13 billion this year, and that the negative impact on exporters such as Mexico "can be managed within the overall framework of the debt initiative we have proposed."

However, it was learned that Mexican Finance Minister Jesus Silva Herzog told the Interim Committee that the lending nations should make some interest rate concessions to countries such as Mexico hard hit by declining oil prices. Several participants indicated they were impressed by "the moderation" of Silva Herzog's comments and demands.

Baker also said that the dip in short- and long-term interest rates since early 1985 would reduce debt service payments by at least $11 billion annually, and that the higher growth and lower inflation rates in the industrial world would add $5 billion to the developing nations' nonoil exports while cutting their import costs by $4 billion.

He said that the total of net new financing envisioned by the Baker plan -- which he now estimates at $40 billion during the next three years, equally divided among commercial banks and the multilateral development banks -- is "not set in concrete."

Meanwhile, although further interest rate cuts on a coordinated basis were discussed Tuesday at both the Group of Five and Group of 10 meetings, it was not clear when or how such action might be taken. A Japanese spokesman yesterday denied reports that another discount rate increase by the central bank in Tokyo was imminent, and West German officials said there is no reason for a further cut in rates.

Other sources, however, suggested that another round of rate reductions is inevitable, largely because of lower oil prices and lower inflation. Japanese Finance Minister Noburo Takeshita yesterday told reporters that he and Baker had agreed, as they had in London in January, that global economic conditions are still favorable for lower interest rates.