The possible influence of caffeine on maxillary structure was studied. Seventeen pregnant rats at days 9 of gestation were randomly divided into two groups. The dams of group 1 received a 20% protein diet ad libitum throughout the experimental period. The dams of group 2 were pair-fed, with group 1, a 20% protein diet supplemented with 2 mg/100 g body weight (B.W.) caffeine. At birth, pups were mixed within the same group and 8 randomly selected pups were assigned to each dam and continuously fed the respective diet. On day 22, 11 male pups from the control and 12 males from the caffeine group were randomly selected, separated from the dams, and continued to be fed their respective diets. On day 44, a rubber elastic band was inserted between the first and second maxillary right molars. The size of the elastic band was increased throughout the next 5 days. Animals were sacrificed at day 49 and the composition of the maxillas was analyzed. After pulverization, organic and inorganic contents of the bones were measured. Zinc (Zn) and hydroxyproline concentration of the caffeine group showed a significant decrease when compared with those of the controls. However, Ca, P, Mg, and hexosamine concentration showed no difference between the groups. The interdental space measured occlusally and laterally with the visual method, and occlusally in histological sections showed no significant difference between the control and caffeine groups, although variation of the space in the caffeine group was less than in the control group.(ABSTRACT TRUNCATED AT 250 WORDS)