Rotifers of the species Mytilina brevispina var redunca were cultured at 24°C under standard and aseptic culture conditions in artificial saline media of three calcium concentrations. 1. All populations were homologated with respect to maternal age by a process of egg selection carried out over three generations. 2. Observations on survival, growth and egg production were made on populations cultured on the three media containing a-full and reduced diet, while the effects of transfer between culture media, periodic washing in chelating agents, and constant exposure to antioxidants were tested on survival and egg-laying in Control cultures. 3, The calcium content of rotifers was investigated by continual exposure to the radionuclide 45 calcium in the three culture media, while the 45 calcium accumulated and withdrawn from rotifers cultured on Control 45 calcium medium and subjected to treatment with the chelating agent sodium citrate, and the 45 calcium accumulated in rotifers cultured on Control 45 calcium medium containing the antioxidant B.H.T., were also investigated. The 45 calcium intake of algae cultured for one day on Control 4-5 calcium medium at the concentration normally employed for experiments was noted. 4. In the case of rotifers cultured on a full diet in the three calcium media the greatest longevity, egg' total, and reproductive period was recorded in the Low calcium cultured population, while the lowest longevity value and total number of eggs laid was recorded in the High calcium group. Continual exposure to 4-5 calcium in each of the three media, revealed an accumulation of calcium that began in all cases at the end of the period of growth in size. This accumulation occurred at a rate that was inverse.ly related to longevity and directly related to the level of radionuclide in each medium in the manner predicted by the Lansing hypothesis. The total 45 calcium taken up in the daily intake of algae in 4 day old rotifers cultured on Control medium approximately the same order as the maximum rate of 45 calcium accumulation in untreated rotifers cultured on the same medium, suggesting a possible source of calcium accumulation in the event of a breakdown in the mechanism of excretion. 5. Rotifers cultured on a reduced diet in each of the three media showed an increase in longevity, egg total and reproductive period compared with rotifers cultured in the corresponding media on a full diet. However, the relative differences with respect to these characteristics were approximately similar between populations cultured on the same dietary level, with the Low calcium cultured population showing the greatest increase in longevity, total egg production and reproductive period. No execution in the growth period or difference in final size was noted in the case of the dietary restricted groups (cf McCay's starvation studies). 6. The results of the culture transfer experiments in which rotifers were transferred to another of the three media at the end of the growth period, revealed that it was the medium on which rotifers were cultured after growth that exerted the major influence on longevity value, total egg production and length of reproductive period. This result is in agreement with the appearance of an ageing factor at the end of the period of growth size postulated in the Lansing ageing theory.