1. The haemolymph of the American cockroach, Periplaneta americana, has been analysed for Na+, K+ and Ca2+ ions in different experimental conditions.
2. The ionic content of the haemolymph is maintained constant after the first larval instar. It is characterized by a rather high K+ level and a particularly large variability which could be related either to the nature of the diet (lettuce) or to numerous alterations of the blood and tissues in relation to the moulting cycle.
3. The regulation of this ionic content when the animal is given different diets is particularly effective. Starvation results in a decrease in the mean ionic concentration in the blood, this decrease following a pronounced increase of the Na+ and K+ levels when starvation is associated with dehydration. The blood K+ level can be raised artificially with high K+ diets (lettuce leaves or high K+ saline) but is then very variable. These results are discussed in terms of absorption, excretion and storage mechanisms.
4. The analysis of blood samples collected successively from the dorsal thorax of different individuals shows that, when the animals are fed normally, the ionic composition varies widely, showing that in these conditions the haemolymph is not homogenous.
5. Serial measurements of blood ions in different regions of the animal show unequal distribution of Na+, Ka+ and Ca+. This result is discussed in view of the very special circulatory system of insects when compared with that of vertebrates.
6. The overall picture of insect blood which can be drawn from these experiments is that the complexity of the regulatory system together with the fact that the circulatory system is open and lacunous allows relatively wide variations in the ionic concentration to occur on both sides of a genetically fixed mean. This particular feature must be taken into account when one considers the physiological role of blood ions in mechanisms such as excitation, conduction or muscular contraction.