December 30, 1986

WHEN Gov. James J. Blanchard of Michigan signed a bill into law last week creating a statewide tuition-guarantee program, Michigan became the first state to adopt an idea that more than a dozen private colleges are already using to help parents cope with the crunch of college expenses. Under the Michigan law, which Mr. Blanchard proposed in January, the state guarantees a child's tuition at any public college or university in the state if parents deposit as little as $3,000 in a new state-sponsored trust fund. How much they will be required to deposit to qualify will depend on the age of the child, with infants' parents paying the least and teen-agers' parents paying the most because they will go to college sooner. The program covers the University of Michigan and 14 other four-year state colleges, as well as 29 two-year schools.

The dollar drifted to a six-year low against the West German mark yesterday and its lowest level in several months against the Japanese yen in thin year-end trading, With most traders on the sidelines, volume was at best one-quarter to one-third the normal level, foreign exchange dealers estimated. ''Most of the activity has been in the dollar-yen market because the Japanese trading companies and real estate investors are cleaning up their books for the end of the year,'' said Christopher Bourdain, a dealer who handles corporate foreign exchange business in New York for the Bank of America. ''The dollar is just drifting against everything else. Nothing is pushing it but nothing is stopping it either.''

New York City, faced with a growing number of homeless families, has been increasing its use of New Jersey hotels to provide them with shelter. In September, the Koch administration began by sending 20 families to a Newark motel, but Newark Mayor Sharpe James objected. A few weeks later, New York City said it would put no more than 40 families in that motel. Since then, city official have referred families to a number of hotels and motels in other parts of New Jersey.

ANTHONY TARABOCCHIA Jr. is an athletic, outgoing boy, normal in every way, except that he was born a dwarf. ''Since birth,'' said his mother, Mary, ''my goal has been to find something for him. I thought if they could transplant hearts and send people to the moon, they could help Anthony grow.'' She took him to doctors for hormone injections when he was 2 and to Lourdes for the baths, but still Anthony stayed small. Then she heard of an operation being done in Europe that could add a foot of height. She asked a dozen American doctors, but could find none to do it, because they felt it too painful and risky.

THE human brain converses with the entire universe, but it maintains a strange and stringent isolation from its own body. Many substances that circulate in the blood hardly enter the brain at all. Some chemicals in the brain will not diffuse outward into the general circulation. This strange selectivity is vital to the brain's proper function. But recently scientists have learned much about how to manipulate it to help the whole body fight disease.