Four months ago, the Turtle Back Zoo was on the brink of extinction. Now business is booming at the zoo, which is owned by Essex County, and it will stay open at least until the end of the year, thanks in large part to an overhaul and the efforts of volunteers from the New Jersey Zoological Society.

     In February, County Executive James W. Treffinger said that the zoo had lost more than $1.2 million in 1995 and that the county could no longer afford to subsidize it. But the zoological society began a drive to save the zoo, raising money, offering volunteer help and persuading the county to attract new sources of revenue. Attendance and revenue are up sharply at the zoo since Memorial Day weekend, compared to this time last year, Joanna Noonan, a spokeswoman for the county, said yesterday. TERRY PRISTIN