We evaluated whether environmental tobacco smoke exposure in utero and/or postnatally affects the biochemical composition of the brain. Pregnant Sprague-Dawley rats were exposed to filtered air (FA) or to sidestream smoke (SS) for 4 h/d, 7 d/wk from d 3 of pregnancy until delivery, then their female pups were exposed to either FA or SS for 9 wk postnatally. This resulted in four exposure conditions: in utero FA followed by postnatal FA (FA/FA), in utero FA followed by postnatal SS (FA/SS), in utero SS followed by postnatal FA (SS/FA), and in utero SS followed by postnatal SS (SS/SS). After completion of the exposures, the brains were removed and divided at the pontomesencephalic junction into forebrain and hindbrain; each specimen was then analyzed for DNA, protein, and cholesterol concentration. Data were analyzed by 2-way analysis of variance. In utero SS had no effect on these three biochemical measurements. However, postnatal SS reduced hindbrain DNA concentration (an indicator of cellular density) by 4.4% (p = 0.001). In addition, the hindbrain protein/DNA ratio (an index of cell size) was increased in these animals by 8.4% (p = 0.001). Hindbrain weight was not affected by SS exposure, but body weight was reduced by 6.4% (p = 0.016). These data suggest that postnatal exposure to SS affects the hindbrain (a region which undergoes significant postnatal growth) by reducing the total number of cells and by increasing cell size. Hindbrain cellular hypertrophy may help offset the decrease in cell number, thereby leaving hindbrain weight unchanged. Despite preserved hindbrain weight, these effects of postnatal exposure to SS may result in neurologic dysfunction.