This study evaluates the maternal immunological response at different stages of gestation in the mouse. This response has been measured by the local G.V.H.R. of lymphocytes from uterine draining lymph nodes and spleens. The results reported here show that the maternal immunological reaction-against paternal antigens carried by the foetus-is lower at the beginning of gestation (on days 6-7) and increases significantly during the last period between 14 and 18 days. In contrast, the temporal G.V.H.R. response of maternal splenocytes is inverted, with lower values from day 14 to day 18.