Mental rotation involves transforming a mental image of an object so as to accurately predict how the object would look if it were rotated in space. This study examined mental rotation in male and female 3-month-olds, using the stimuli and paradigm developed by Moore and Johnson (2008). Infants were habituated to a video of a three-dimensional object rotating back and forth through a 240 degree angle around the vertical axis. After habituation, infants were tested both with videos of the same object rotating through the previously unseen 120 degree angle, and with the mirror image of that display. Unlike females, who fixated the test displays for approximately equal durations, males spent significantly more time fixating the familiar object than the mirror-image object. Because familiarity preferences like this emerge when infants are relatively slow to process a habituation stimulus, the data support the interpretation that mental rotation of dynamic three-dimensional stimuli is relatively difficult--but possible--for 3-month-old males. Interpretation of the sex differences observed in 3- and 5-month-olds' performances is discussed. (Contains 3 figures.)