With mortgage interest rates plummeting, this is a good time to think about refinancing if your current mortgage is higher than 11 percent or if you have an adjustable rate that makes you uneasy.

You can find 30-year fixed-rate mortgages as low as 9.25 percent in many areas and rates under 10 percent throughout the country.For example, in California, where rates are usually among the highest, California Home Federal is offering a fixed rate of 9.87 percent with 1-1/2 points.

More typical for many areas is the 9.5 percent with 1 point offered by First Union Mortgage, headquartered in North Carolina.

And Citicorp Mortgage has a rate of 9.3 percent with 3 points in many locations.

If you think you might benefit by refinancing, find out what the lowest rates are in your area. Newspapers often run a weekly chart showing mortgage rates, and companies with competitive offerings frequently advertise. You might also telephone several lenders to compare rates.

It's worth the trouble because a two-percentage point spread in mortgage rates makes a tremendous difference over time.

For instance, you'd pay monthly principal and interest of $923 on a $95,000 mortgage with an interest rate of 11.25 percent. The payment would drop to $781 with an interest rate of 9.25 percent, a difference of $142 a month.

That $142 adds up to $1,704 in just one year. Over five years, you would save $8,520.

Another possibility when interest rates are falling is to trade in a 30-year mortgage for a 15-year mortgage without significantly raising your monthly costs.

On a $95,000 mortgage to be paid off over 30 years, the monthly principal and interest payments at 11.25 percent are $923. But you could pay off the mortgage in 15 years for $978 a month if the interest rate dropped to 9.25 percent.

In other words, for just $55 a month more, you could pay off your mortgage in half the time with the lower interest rate. Unfortunately, refinancing is expensive, so before you plunge in, you need to make sure your savings will be substantial.

As a rule of thumb, economists say you should be able to recoup your refinancing costs within two years through the money you save on a less expensive mortgage.

But that formula doesn't suit everyone. If you're planning to move in three or four years, refinancing might not be worth the hassle. On the other hand, if you think you'll stay in your house for a decade, refinancing might be worthwhile even if it took you three years to pay off your closing costs.

While refinancing costs may amount to between 2 percent and 4 percent of the mortgage, most lenders will let you wrap most of the expense right into the mortgage. That way you don't need a lot of cash upfront.

Banks and savings and loans will be happy in this slow season to sit down with you and figure out how much you would save by refinancing and what the closing costs would be.

Or you can figure out the difference in mortgage payments yourself:

- Write down what you pay each month in principal and interest. (Don't include the portion of the payment that goes for taxes and insurance. You can find out how much taxes and insurance are from your annual escrow statement.)

- Find out what rates are being offered in your area, and use the following factors (these are the numbers commonly used by lenders):

9.25 percent interest - factor 8.23

9.50 percent interest - factor 8.41

9.75 percent interest - factor 8.60

These factors represent the monthly principal and interest you'd pay for every $1,000 you borrowed on a loan to be repaid over 30 years. So, to find out what monthly payments would be on a $95,000 30-year mortgage at 9.25 percent interest, multipy 95 by 8.23. The answer is $781.85.

- Subtract the monthly payments on the lower interest rate from your current mortgage payment. The answer shows how much you'd save each month with the lower interest rate.