The membranes of kidney cells of 3-spined sticklebacks were examined in freeze-etch replicas. The numbers of particles adhering to surfaces and fracture faces of the outer cell membranes and the membranes of the basal labyrinth were determined. The latter membranes probably are the main location of ion-transporting enzyme complexes.

The total number of particles per cell in freshwater fish exceeds that of seawater fish by about 50 % for the outer cell membrane, and by almost 200 % for the membranes of the basal labyrinth.

After transfer of seawater fish to freshwater, particle numbers increase and their densities approximate freshwater values after 20 h. This rise in particle numbers coincides with the increase of ion-transporting activity of the cells known to take place after transfer to freshwater.

The rate of increase of particle densities is enhanced after injection of ovine prolactin. This hormone is known to stimulate Na+/K+-ATPase activity of the basal labyrinth of teleost kidney cells.

The results indicate that the particles represent enzyme complexes. The number of particles is probably under hormonal control. The increase in particle densities after transfer to freshwater is accompanied by a rise in the number of nuclear pores, which is noticeable by 10 h.

No changes were observed in the density of the particles adhering to the fracture faces of gap junctions.

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