Crosses between certain Drosophila melanogaster strains may give rise to female sterility of non-Mendelian determination. Reduced fertility is observed in F1 females, known as SF females, from crosses between females of "reactive" strains and males of "inducer" strains. The extent of this reduction of fertility depends on the strains which are used in the cross and on two non-genetic factors: age and temperature. The fertility of SF females increases with ageing. Also, exposing them for a short period to a high temperature (29 degrees C) either increases or decreases the probability of hatching of the eggs according to the stage of oogenesis at which the heat treatment is applied. A very striking point is that qualitatively quite similar, though attenuated, effects are observed when the two factors (ageing and temperature) are applied not directly to SF females, but to their maternal ancestors: mothers and grandmothers.