Breeding data for the period 1943 to 1960 recorded at the United States Rabbit Experiment Station at Fontana, Southern California, were analysed to ascertain seasonal variation in several reproductive traits of New Zealand White rabbits. There were 4 months (June, July, August and September) when the average daily maximum temperature was higher than 80° F. Averages for the traits analysed were: conception rate, 68%; total litter size at birth, 7·39; number born alive per litter, 6·74; proportion of stillbirths, 8·85% of total born. Conception rate exhibited a highly significant seasonal trend from a high in the spring to a low in the autumn. Total litter size at birth, and number born alive per litter, decreased consistently from the spring to a low in September. The monthly proportion of stillbirths closely paralleled changes in maximum temperature in litters with at least one survivor, but not in litters with no survivors.