David and Dorothy Diesing were a model couple. They rented their own apartment, they managed their own finances, and six months ago had their first child, a daughter, Stephanie.

For David, 30, and Dorothy, 37, all this was a struggle. Both are mentally retarded.

But now the Diesings are engaged in a drawn-out court dispute to try to regain custody of their daughter. A judge will decide within a month whether David and Dorothy Diesing, one of Michigan's first mentally retarded couples to marry, are as model as parents as they are as citizens.

The judge's decision could make the Diesings' battle a test case in this state, which since 1974 has had one of the nation's most progressive laws dealing with mentally retarded citizens. It also could test a concept that has been gaining nationwide currency: "mainstreaming," or integrating mentally retarded citizens into the general community.

In Michigan, "mainstreamed" retarded people may drive, enter contracts and marry. At issue in the Diesing case, in effect, is the right to be parents.

But Ron Gudenau, the Detroit police officer who removed Stephanie from her parents' home last fall, sees the case differently. Of equal importance, he says, are the rights of the child.

The custody struggle was triggered in October, when the couple was told by their caseworker to report for dental checkup. Unable to find a babysitter, and unwilling to expose the infant to Michigan's chilling fall winds, the Diesing fed her, changed her diaper and put the side of her crib fully up.They also locked a pet dog inside the room to watch over "and to protect" Stephanie, while they went to the dentist.

When they arrived at the clinic, they were asked about Stephanie. When they replied that she was home alone, police were called. Stephanie was removed from the home and has since been in the custody of David's mother, Bernice Ferry.

The Diesings' case is complicated because when the 2-month-old infant was found by police, she weight only six pounds and showed signs of malnutrition. John Devers, the Diesings' court-appointed attorney, insists that the infant's weight problems were due to her small weight at birth (4 pounds, 12 ounces) and to a possible allergy to her formula.

The formula has been changed twice since Stephanie was removed from the home.

David Diesing's mother and the couple's caseworker, Nancy Keyt, have refused to comment on the case. They are not parties to the state-brought suit, and no testimony has yet been called by the judge, Robert E. Spier of McComb County probate court.

Michigan's mental health code includes one chapter exclusively devoted to the rights of the mentally retarded. It guarantees basic equal rights for mentally impaired citizens, including the rights to live their lives with little or no interference from the state. Only the courts can restrict those rights, and then only with the burden of proof upon those who would take them away.

"Mentally retarded citizens are presumed to be as functionally capable as you or I until it is proven otherwise," said attorney Devers. He said the law "brought us into the 20th century" with respect to the mentally retarded.

But the Diesings' case underscores some of the problems and perils confronting retarded citizens as they try to adapt and live their lives normally in society.

The Diesings now have enrolled in classes "to learn how to be good parents." They would have taken the classes earlier, Dorothy said, if they had known classes were available.