President Reagan yesterday signed a trade bill extending tariff concessions to 140 developing countries but vetoed several other measures, including a bill creating an American Conservation Corps to provide jobs for up to 37,000 young people on public lands.

Reagan also vetoed legislation creating two new institutes -- for nursing and for arthritis -- at the National Institutes of Health and a bill extending medical loan programs and providing funding for community health centers for the poor, especially in rural areas.

Reagan said he vetoed the legislation because it was too expensive, changed regulations unnecessarily or involved government in matters best left to the private sector. But supporters of the highly popular bills said some of the vetoes showed that Reagan was intent on ignoring the nation's neediest.

Sen. Orrin G. Hatch (R-Utah), a conservative supporter of the administration, had urged Reagan to sign the two health bills, saying they were vital to the "safety net."

Rep. Henry A. Waxman (D-Calif.), noting the large bipartisan majorities that passed the health bills, said yesterday that "this is a perfect example of the president's indifference and opposition to programs that affect the health and welfare of the American people."

Reagan yesterday signed 64 bills into law. They ranged from a new federal policy on cable television and an extension of the Head Start program to the naming of a federal center in Elkins, W. Va., after retiring Democratic Sen. Jennings Randolph.

In a statement accompanying his veto of the conservation corps, Reagan said the legislation was "based on the discredited approach to youth unemployment that relies on artificial public-sector employment . . . . I believe that America's unemployed youth would be better served by reducing federal spending . . . . "

In addition, he said, the corps "is not a necessary or effective way of managing federal lands."

The conservation corps would have set up a $225 million program to employ youths in state and U.S. parks and on Indian lands.

Sponsors of the measure and environmentalists reacted angrily yesterday to the veto. Sen. Daniel Patrick Moynihan (D-N.Y.), a chief sponsor, said, "I'm disappointed. We'll be back."

Diane MacEachern, a spokesman for the Sierra Club, said Reagan was "blowing a perfectly good opportunity to put people to work and clean up the environment . . . . We've been saying for four years that . . . Reagan cares nothing about cleaning up the environment, and I think . . . he's proven us to be right."

Reagan said the $2.9 billion multi-year health-services authorization bill was "seriously flawed." He said that its price tag was too high and that it would have increased medical school loan programs that he believes are "obsolete."

In addition to reauthorizing loan programs for facilities that train health professionals, the measure would have reauthorized community and migrant health-care centers and extended the National Health Service Corps, for which health professionals work to repay U.S. scholarship aid. Reagan opposed the NHSC.

Reagan said the bill "takes the wrong approach to health-professions training" and "continues excessive taxpayer subsidies . . . . "

Hatch wrote to Reagan two weeks ago urging him to sign the measure, saying it "is truly a safety net to . . . help those who cannot help themselves."

Reagan also vetoed a $4.7 billion NIH authorization, which provided the first comprehensive statutory revision of NIH authority in 40 years.

In addition to establishing nursing and arthritis institutes, the legislation would authorize studies of ethical questions in medicine and of so-called "orphan" diseases that affect relatively few.

Reagan said his veto would not affect the NIH's funding for this fiscal year since money to fund the agency was included in the omnibus spending bill that Congress passed just before it adjourned and that Reagan signed.

But, he said, "the NIH has stood as an example of excellence for 40 years. I do not believe it . . . necessary or wise to revise . . . the laws under which it has so successfully operated."

Other bills Reagan signed yesterday included:

* A politically popular measure to guarantee a January cost-of-living increase to Social Security recipients regardless of the inflation level. Current law requires that inflation be above 3 percent to trigger a benefit increase. When the legislation was first proposed by the administration, inflation appeared to be staying below the 3 percent mark. It has since moved above 3 percent.

* A measure extending the Head Start and low-income, energy-assistance programs for two more years and creating a program using schools after hours to care for "latchkey children."

The bill also authorized two more years of funding for the Community Services Block Grants, the Community Food and Nutrition program and the Native American programs, which provide economic development assistance and social services for Indians.

* A bill allowing governors to ask the secretary of transportation to ban extra-long and extra-wide trucks from using older, less safe sections of the interstate highway system.

* A cable television bill, which would reinforce the right of cities to control local cable franchising and demand certain cable television services.