The effect of nutritional perturbations upon growth and catch-up growth in mass in the rat have been reevaluated by interpretation of published data. The earlier described cellular mechanisms involved were prematurely reported to be invalid. Hypertrophy and hyperplasia have been separately assessed by employing studies providing DNA content of the body or organs permitting calculation of number of "DNA units" and mean size of the units. Changes in unit number and size accounted for reported changes in gross mass. Perturbation during the periods of gestation and/or nursing decreased or increased the number of units without changing the size of the units, and in these cases the change in body or organ mass persisted into maturity. Perturbation during adulthood changed size of the units without changing their number, and in these cases catch-up occurred. Post-weaning was a transitional period with perturbation sometimes changing unit number after which the change persisted and sometimes unit size after which catch-up occurred. Failure in most cases of DNA unit number and unit size to change simultaneously led to speculations about the mechanisms involved. Besides perturbations resulting from experimental interventions, the rat is influenced also by circumstances with unpredictable nutritional impact encountered during the ontogeny of free-living individuals, for example, interactions between pups and mother and between siblings. Presumably the generalizations arrived at above also apply in these cases.