Macronuclei isolated from Tetrahymena are contracted in form (average diameter:10.2 /.tm) at a final Ca/Mg (3:2)concentration of 5 raM. Lowering the ion concentration to 1 mM induces an expansion of the average nuclear diameter to 12.2/xm. Both contracted and expanded nuclei are surrounded by a largely intact nuclear envelope as revealed by thin-sectioning electron microscopy. Nuclear swelling is accompanied by an expansion of the nuclear envelope as indicated by the decrease in the frequency of nuclear pore complexes from 52.6 to 42.1 pores/ /zm ~ determined by freeze-etch electron microscopy. Contracted nuclear membranes reveal particle-devoid areas (average size: 0.21 /xm 2) on 59% of their fracture faces at the optimal growth temperature of 28~ About three-fifths of the number of these smooth areas disappear upon nuclear membrane expansion. Electron spin resonance using 5-doxylstearic acid as a spin label indicates a higher lipid fluidity in contracted than in expanded nuclear membranes. Moreover, a thermotropic lipid clustering occurs at -17~ only in expanded nuclear membranes. In contrast to the nuclear membrane-bound lipids, free lipids extracted from the nuclei rigidify with increasing Ca/Mg concentrations. Our findings are compatible with the view that the peripheral layer of the fundamental nuclear protein-framework, the so-called nuclear matrix, can modulate, inter alia, the lipid distribution and fluidity, respectively, in nuclear membranes. We suggest that a contraction of the nuclear matrix's peripheral layer induces a contraction of the nuclear membranes which, in turn, leads to an isothermic lateral lipid segregation within nuclear membranes. KEY WORDS nuclear envelope nuclear membranes nuclear matrix membrane lipid fluidity isothermic membrane lipid segregation Biomembranes are widely considered as being 'fluid' entities, i.e., membrane integral proteins can diffuse laterally and/or rotationally within a fluid lipid bilayer continuum (41). Recently, evidence has accumulated that the mobility of membrane integral proteins can be controlled, inter alia, by membrane-associated skeletal proteins (e.g., references 11, 37, 42, 53).J. CELL BIOLOGY ~ The Rockefeller University Press 9