Abstract 1. 1. Sodium, potassium, calcium and magnesium have been determined in the blood of hydrated specimens of Helix pomatia, Cepaea nemoralis, C. hortensis, Arianta arbustorum, Archachatina marginata, Arion ater ater, A. ater rufus, Lymnaea stagnalis and Planorbarius corneus, and the average values, together with others already published, have been compared. 2. 2. Such averages for sodium, potassium and calcium each vary by a factor of 2–2·5 while those for magnesium vary by a factor of 12. 3. 3. There is a positive correlation between the average concentrations of calcium and magnesium. 4. 4. In the Stylommatophora, there is a negative correlation between the average concentrations of sodium and magnesium, perhaps related to a similarity of osmotic pressure in hydrated specimens.