26 CFR §1.265-3

1. (a)In general. No deduction is allowed to a shareholder of a regulated investment company for interest on indebtedness that relates to exempt-interest dividends distributed by the company to the shareholder during the shareholder's taxable year.
2. (b)Interest relating to exempt-interest dividends.
   1. (1)All or a portion of the interest on an indebtedness relates to exempt-interest dividends if the indebtedness is either incurred or continued to purchase or carry shares of stock of a regulated investment company that distributes exempt-interest dividends (as defined in section 852(b)(5) of the Code) to the holder of the shares during the shareholder's taxable year.
   2. (2)To determine the amount of interest that relates to the exempt-interest dividends the total amount of interest paid or accrued on the indebtedness is multiplied by a fraction. The numerator of the fraction is the amount of exempt-interest dividends received by the shareholder. The denominator of the fraction is the sum of the exempt-interest dividends and taxable dividends received by the shareholder (excluding capital gain dividends received by the shareholder and capital gains required to be included in the shareholder's computation of long-term capital gains under section 852(b)(3)(D)).