Alan Rosenthal, a man who has made a career of studying state legislatures, looked at a group of bleary-eyed state senators and representatives yesterday morning and said, "You've been asking for new federalism for years. All I can say is, you've asked for it, you've got it and you deserve it."

The laughter that greeted Rosenthal's sarcastic pronouncement was nervous because, as Rosenthal put it later, "It's difficult for legislators to figure out exactly what all this means because no one knows yet. We may not know for a while."

Which is why the approximately 300 legislators, staff members and business people who gathered here this weekend for a meeting of the National Conference on State Legislators virtually were unanimous in viewing President Reagan's concept of turning federal programs back to the states with trepidation as well as excitement.

The legislators were here for a meeting of the Assembly on the Legislature, one of two arms of the national legislators group. It tries to establish procedural guidelines for legislatures to use in setting internal policies. The other, the State Federal Assembly, addresses federal problems that affect state governments.

Even though this gathering was set up so the assembly's seven committees could finish policy statements that will be presented at its parent group's annual meeting this summer, much of the talk was about federalism and what it may mean to the 7,500 elected people who work in state legislatures.

"I think most of us like the idea of federalism. I know I do," said Benjamin L. Cardin, Speaker of the Maryland House of Delegates. "But we all know how tight money is already and we're all concerned with what kind of changes the added responsibility might bring to our legislatures."

Cardin, like most of his colleagues, strongly opposes the idea of full-time legislatures. But with the added responsibilities federalism would bring, many states, including Maryland, might have to consider the possibility.

"People say we should be more like Congress and meet all the time," said Cleta Deatherage, a legislator from Oklahoma. "I think Congress should be more like us. I think it should adjourn on Sept. 30 every year, except in the case of a national emergency--perhaps especially in case of a national emergency.

As always, though, the main cause for concern was not time or the concept of expanding staff responsibilities, but money--or, more specifically, the lack of it.

"We have a list of 124 programs which would come back to the state under the federalism plan," said Richard Schneller, majority leader of the Connecticut Senate. "Probably, at some point, we're going to have to decide which ones to cut, which ones to eliminate."

Most of the legislators agreed they would feel more comfortable if more information on federalism was forthcoming from Washington, if the White House could be definitive about what will be expected of the states.

Rosenthal, however, cautioned all not to expect too much too fast. "This is a difficult period for all of you," he said. "You may be prouder under new federalism, but you'll be poorer, too.

"But look, this debate is going to for a while. It should go on. What all this means and does will be discussed for another five years, maybe 10 years. At least for a while, everything is going to be in flux."

Ross Doyen, of Kansas, president of the conference, agreed. "It's a challenging time for all of us. We're going to have a lot of new responsibilities."