The total number of oocytes in the ovaries of the rat decreases with age, the relation between the two variates conforming to the exponential relation y = axb. The regression of oocyte numbers on age is defined by the equations log y = 4·334 − 0·378 log x for the figures provided by Arai [1920], and log y = 4·561 − 0·476 log x for our own observations, x being measured in days. The two values of 'a' and of 'b' do not differ significantly from each other.