By a vote 7 to 5, the D.C. City Council gave preliminary approval last week to a bill that would permit adoptees over the age of 21 to open birth records and learn the identities of their natural parents.

Under the bill, natural parents could refuse to allow their identities to be disclosed. Final council action on the bill is scheduled for May 1.

The roll call on the measure, Council Bill 3-19, was as follows:

For the bill: David A. Clarke (D-Ward 1), Arrington Dixon (D-Chairman At Large), Betty Ann Kane (D-At Large), Hilda Mason (Statehood-At Large), John L. Ray (D-At Large), Polly Shackleton (D-Ward 3) and John A. Wilson (D-Ward 2).

Against the bill: Willie J. Hardy (D-Ward 7), Jerry A. Moore Jr. (R-At Large), Wilhelmina J. Rolark (D-Ward 8), William R. Spaulding (D-Ward 5) and Nadine P. Winter (D-Ward 6).

Winter introduced an amendment, rejected by the council, that would have made the bill apply to future adoptions, but not those in the past.

For the amendment: Hardy, Rolark, Spaulding and Winter.

Against the amendment: Clarke, Dixon, Kane, Mason, Moore, Ray, Shackleton and Wilson.

The council rejected an amendment proposed by Rolark to the 1979 supplementary budget, Bill 3-118, that would have eliminated $4 million to contruct a sludge composting facility at Blue Plains.

For the amendment: Kane, Mason and Rolark.

Against the amendment: Clarke, Dixon, Hardy, Moore, Ray, Shackleton, Spaulding, Winter and Wilson.

The council also rejected an amendment to the supplementary budget, proposed by Kane, that would have eliminated $150,400 in administrative funds for the D.C. Convention Center.

For the amendment: Kane, Mason.

Against the amendment: Clarke, Dixon, Hardy, Moore, Ray, Rolark, Shackleton, Spaulding, Winter and Wilson.