The District of Columbia City Council gave preliminary approval last night to a bill that would permit adults who were adopted in childhood to seek to obtain the names of their natural parents.

The 7-to-5 vote in favor of the measure followed more than 90 minutes of sometimes emotional debate. The measure is similar to one that received preliminary approval last November only to be killed by a tie vote on final consideration later that month. The pending bill requires a second approval before final passage.

Under current city law, birth records are normally sealed and are not available for inspection. The pending bill would permit adoptees who have reached 21 to apply to the D.C. Department of Human Resources for the right to see birth records. However, the department would be required to attempt to contact parents, who could insist that their identities be kept secret.

A campaign to open birth records to adoptees culminated in a hearing last year before a council committee. Many witnesses said they wanted the right to trace their roots or to learn important medical information about themselves.

The action came at a council meeting that lasted until after 1:30 a.m. today. In other action, the council gave unanimous final approval to a bill that will permit taxicabs to carry advertising signs on their roofs, and voted to extend a 3-year moratorium against conversion of apartment buildings to cooperatives ownership for another 90 days. Permanent legislation on the matter is pending.