Standard & Poor's Corp. yesterday lowered the ratings on McDonnell Douglas debt, the third such reduction since last year, citing cost pressures on military programs and the recent cancellation of the $57 billion Navy A-12 attack jet program.

The St. Louis-based aerospace company's senior debt was reduced to a triple B rating -- the lowest rating still considered "investment grade." Many institutional investors have bylaws requiring that they hold only investment grade securities.

The downgrading affects all of McDonnell's $5.6 billion in debt, including that issued by the company's financial services unit. The aerospace side of the company holds $2.06 billion in long-term debt and $909 million in short-term debt, primarily in the form of bank loans.

In issuing the downgrade, S&P; cited the $1.3 billion reimbursement that the Navy has demanded from McDonnell and its partner General Dynamics as a result of the A-12 termination.