1. Measurements have been made of a number of basic parameters related to osmoregulation in the ammocoete larva of the brook lamprey (Lampetra planeri Bloch).
2. There is no significant variation in the water content of whole animals or their tissues between animals analysed during November and July, but the fat content rises in July and the majority of fat storage takes place in the muscles.
3. Analyses have been made of the major osmotically active ions in whole animals, in muscle and in blood. Unlike previous investigators we found no significant seasonal variation in any of these components. The freezing-point depression of ammocoete blood (113-119 mM/l. NaCl) is lower than that of adult river lampreys (140 mM/l. NaCl), but apart from higher potassium values, monovalent ions contribute nearly the same proportion to the total osmotic pressure in both animals. The muscle analyses indicate that the ammocoete has a higher ratio of potassium to sodium (14.8:1) than other vertebrates.
4. Measurements of the fluid and ionic compartments have been made on whole animals and on muscle using isotopic dilution techniques and conventional analyses. The extracellular space of muscle is about half that of whole animals (24.4 % body weight) and contains all the chloride and 69 % of the sodium within whole muscle. Potassium almost completely replaces sodium within the cells and part of this may be bound sodium in some tissues.
5. The urine output of the ammocoete amounts to 198 ml. per kg. per day and is similar to the values previously obtained from other lampreys in fresh water.
6. The high potassium to sodium ratio within cells, together with the lack of seasonal variation and low variability in ionic analyses, is taken as an indication of the presence of very efficient controlling mechanisms operating on extracellular fluids and within the cells themselves.