Fifty-four male and forty-five female 7-week-old rats, Wistar strain, average weights 190 g and 140 g respectively, were randomized by weight into three groups of eighteen rats each (males) and three groups of fifteen rats each (females) and fed on a semi-purified diet containing (per kg) 2 (low), 5 (normal) or 20 g (high) Ca as CaCO3 for 2 weeks. Each group was then further randomized into three groups of six rats each (males) and five rats each (females) and given a meal (10 g of the same diet) containing either 2, 5 or 20 g Ca as 47CaCO3. 47Ca was determined in quantitative daily collections of faeces over 7 d and fractional absorption of 47Ca estimated by extrapolating the linear portion (days 3-7) of the plot of log 47Ca retention v. time back to the time of isotope administration. Absorption of meal Ca was higher in males than in females and was affected similarly in males and females by previous dietary Ca intake and meal Ca content. Fractional absorption of meal Ca decreased with increasing previous dietary Ca intake and with increasing meal Ca content, and the combined effect of these two variables caused fractional Ca absorption to vary from 11-89%. Absolute absorption of meal Ca decreased with increasing previous dietary Ca intake and increased with increasing meal Ca content. The influence on Ca absorption of variations in meal Ca content (load effect) was greater than that of variations in previous dietary Ca intake (adaptive effect). These results show that previous dietary Ca intake and meal Ca content are both major determinants of Ca absorption from meals in intact rats fed in the normal way and that the rat responds to these factors in a manner similar to that reported for humans. This study provides further evidence of similarities between rats and humans in dietary Ca absorption.