The objective of this study was to evaluate the body weights up to 18 months of age of 12 breed groups of Zebu (Z), Brown Swiss (BS) and Charolais heifers (CH), and their crosses under tropical conditions. A total of 1434 data on weaning weights adjusted to 240 days (WW8), average daily gain to 240 days (ADG8) and 1025 body weights adjusted to 550 days (BW18) and average postweaning daily gain (ADG18) of heifers born from 1981 to 1995 were used. Cows and calves remained together from birth to weaning and grazed on Guinea grass (Panicum maximum). Years of birth were grouped in three periods, because of the small numbers of observations per year (1981-85, 1986-90 and 1991-95). Similarly, months of birth were grouped in three seasons: dry (February to May), rainy (June to September) and windy and rainy (October to January). Ages of dams were classified in six groups (<or=3, 4, 5, 6, 7 and >or=8 years). Breed groups were animals with 12.5%, 25.0%, 50.0%, 62.5%, 75.0% and 100% BS genes and 25.0%, 50.0%, 75.0% and 100% CH genes and also Z and undefined crossbred animals. Data were analysed using a fixed model that included effects of period, and season of birth, age of dam and breed group of heifer. Analyses of variance showed significant (p < 0.05) effects of period, season and breed group for all traits. However, parity was significant (p < 0.05) only for WW8 and ADG8. Interactions among main factors were not significant (p > 0.05). Z heifers gained the least weight to weaning but were similar to BS at 18 months of age. CH were heavier at 18 months of age than BS heifers, but not at weaning. Body weights at 18 months of age of 1/8 and 1/4 BS crossbred heifers were similar (p > 0.05) and lower than F1 (BS x Z) (p < 0.05); however, the later and higher grades of BS had the highest body weights at 18 months of age and were similar to CH and 1/4 CH x 3/4 Z heifers. Undefined crossbred heifers had better performance than the Z breed group. Environmental factors were important sources of variation for pre- and postweaning traits. Under the conditions of this study, crossbred heifers generally performed better than Z heifers up to 18 months of age.