Nest defense behavior was examined in wild female house mice (Mus domesticus) that were derived from a stock initially trapped in Alberta, Canada. The first objective was to determine whether behavior toward pups prior to mating was related to the intensity of postpartum aggression in a variety of social situations. Therefore, prior to the experiments we screened virgin females for their behavior toward a newborn pup [60% of the females exhibited infanticide and 40% were noninfanticidal: 7% were parental (retrieved and hovered over the pup) and 33% ignored the pup]. Infanticidal and noninfanticidal females were then mated with males and used in four experiments. In Experiment 1 the females were housed individually prior to deliver, while in Experiment 2 the females were allowed to remain with their mates; in both situations all females successfully reared litters of similar sizes. Male and female intruders (that had all exhibited infanticide when previously tested with a pup) were placed separately into a test cage containing a lactating female during the first four days after delivery. Regardless of the presence of the stud male, previously infanticidal females were more aggressive (exhibited more attacks per min) toward both male and female intruders than were previously noninfanticidal females; infanticidal females also exhibited more of both forms of attack (offensive and defensive) and also attacked with greater intensity than did noninfanticidal females. The number of attacks toward intruders of both sexes increased for both infanticidal and noninfanticidal females between Day 1-4 postpartum, but very high rates of attack were observed on all days by the lactating females, including the day of delivery. In Experiments 3 and 4 only the most aggressive (previously infanticidal) females were tested. In Experiment 3, two unrelated, unfamiliar females were mated separately and then were housed together just prior to delivery, which was planned to occur 3-4 days apart. In 5 of the 15 cages, all pups disappeared on the day of delivery of the second female to deliver her litter. In the remaining 10 cages, it appeared that none of the pups produced by the 20 females were killed. Thus, in this experiment, 66% of pups survived to Day 4 postpartum. In Experiment 4, two previously infanticidal female siblings, which had been housed together since birth, were placed together with a stud male. In all 9 cages only one female became pregnant and delivered pups, but only 3 litters survived to Day 4 (no litters were observed being attacked during intruder tests).(ABSTRACT TRUNCATED AT 400 WORDS)