The post-natal development in CF rats from 5 litters was followed anthropometrically from the day of delivery to day 75. Weight, CR length, tail length and patello-calcaneal length of individual rats were measured every day and the rate of gain per day was calculated for each parameter. During this period, the rate of gain in weight showed two peaks (from days 26 to 30 and 56 to 60). The CR length and tail length had only one such peak (from days 16 to 20 and 21 to 25, respectively). The patello-calcaneal length rather exhibited an initial fall in the rate of growth up to day 45 followed by a slight increase till day 50 after which it became steady. Robustness as judged by: formula: see text was found to be maximum in the first few days after birth. Body weight or: formula: see text showed better correlations with other parameters than the gross age. The growth curves appeared as straight lines having breaks, indicating that though the growth was exponential it had sudden changes in its rate.