To determine the nutritional requirment for iron in fish growth, the effect of daily addition of supplemental ferrous sulfate on the growth of Xiphophorus helleri and Xiphophorus maculatus (the swordtail and the platyfish) was measured by determination of weight gain and hematocrit levels. Both variables increased as a result of the added iron. The effectiveness of the treatment diminished as sexual maturity was approached. Treatment with ferrrous sulfate led to an increase in growth rate even in fish fed maximally with live brine shrimp. Ferric salt was not judged effective. Addition of ferrous iron also significantly decreased the mortality from hatching to maturity.

Measurements of oxygen consumption in whole fishes have been difficult to make, because of the number of variables which affect the rate of gas utilization in these forms. By preference a manometric method is used, but in the case of constant swimmers, such as mackerel, it is necessary to measure oxygen depletion by chemcal methods. Data have been reported from a variety of sources, and have been published using a variety of parameters. Tabular data (cf. Heilbrunn, '43) are usually presented in cubic centimeters of oxygen consumed per gram of wet weight per hour, conversions being made from the data presented in the original reports, using the rough approximation that wet weight is four times the dry weight.Although it is widely believed that organic nitrogen provides a better estimate of actively respiring tissue than does weight (cf. Krogh, '41), very few of the tabulated data make any mention of nitrogen content.Data reported on the rates of oxygen consumption in fresh water fishes range from 0.1 cm3/g/hr in the case of the carp, to 0.35 cm3/g/hr in the pike (Krogh, loc. cit.).In order to establish base line respirations for fishes to be used in a comparison between normal and cancerous individuals it became necessary for us to establish norms for individuals of various ages and sizes. The cancerous fish to be employed is a hybrid between the platyfish, Xiphophorus maculatus and the swordtail, Xiphophorus helleri. The two parent species were chosen for our measurements. METHODSFishes were introduced into standard 15 ml Warburg flasks containing 5 ml of water and 0.2 ml 5% KOH in the center well, or into 125 ml flasks containing 50 ml water and 1 ml 5% KOH in the center well. Measurements were made at temperatures between 22" and 26°C. After attachment of the flask to the manometer, the manometer was fitted with an oxygen well constructed of 100 gauge Saran wrap (supplied through the courtesy of the Dow Chemical Corporation) held in place above the stopcock by a rubber band (Barlow, '61). Thus oxygen removed from the ambient water was replaced by oxygen on re-equilibration, rather than by air. Measurements werc made for periods ranging from 24-41 hours, at two hour intervals, to minimize fluctuations occasioned by lack of shaking the flasks. Measurements made in this way indicated that the procedure was successful, as no change in Qoz appeared between the early and later readings.At the conclusion of the measuring period the fishes were removed from the flasks and determinations of Kjeldahl nitrogen were made.On occasion, fish to be utilized were pretreated with small amounts of various additives for two weeks. DPN, TPN, calcium pantothenate, thiamine and riboflavin were added daily at the rate of 2 mg/500 ml of water to the aquarium; ferrous sulfate at the rate of one mg/500 ml, and cupric sulfate at the rate of 0.001 mg/500 ml. RESULTSThe data from the two species utilized are reported in table 1 and table 2. AS may be seen from these tables, a gen-