The influence of corticosteroids on the lipid polymorphism of dielaidoylglycerophosphoethanolamine was studied by 31P NMR spectroscopy and differential scanning calorimetry. Both techniques evidenced two transitions in the pure lipid samples. The first one corresponded to the gel+liquid crystalline phase transition. It occurred at a temperature of 38.9 "C, as measured by differential scanning calorimetry and at 35 -40 "C as detected by 31P NMR. The second transition corresponded to the bilayer-thexagonal HI, phase transition. It occurred at 64.2 "C as measured by differential scanning calorimetry and at 60 "C as detected by NMR. Addition of corticosteroids led to different specific effects on the bilayer+hexagonal HI, phase transition, according to their chemical structure. These effects appear to be the result of low amounts of incorporated steroids, according to binding studies (partition coefficient values range between 5 and 54). The presence of a conjugated 3-keto group in the steroid molecule (progesterone) promoted a downward shift in the bilayer + hexagonal H,,.phase transition temperature by about 6 -7 "C as compared to the 38-OH-bearing compound (pregnenolone), which did not exhibit any appreciable effect. No change in the AH of transition could be measured. The presence of the 21-OH group (like in deoxycorticosterone) induced the formation of a structure, characterized by an isotropic lineshape of the 31P NMR spectrum at temperatures where the 'hexagonal' type of lineshape is present, without steroid. The transition from the bilayer to this other structure occurred at a slightly higher temperature than the bilayer+hexagonal HI, phase transition. It corresponded to a peak in differential scanning calorimetry scans with a AH of 2.1 kJ .rnol-'. The presence of the 17j-OH group as present in 17j-OH-progesterone and 1 I-deoxycortisol suppressed the two former effects. These compounds had no influence on the bilayer-thexagonal H!,phase transition. The additional presence of the 118-OH group like in corticosterone and cortisol, evoked a stabilization of the bilayer organization as the bilayer-thexagonal HI, phase transition temperature is shifted upward by about 10 "C. This was accompanied by a decrease of the AH to 0.8 kJ 'mo1-l. Besides this, the corticosteroids did not affect to a large extent the gel-+liquid crystalline phase transition : a general slight downward shift of the transition temperature and a small broadening of the transition were observed without significant change in the AH. The effects of corticosteroids on the bilayer -+hexagonal HI, phase transition are qualitatively correlated with their influence on biological membranes. Steroid-membrane interactions are also involved in steroid hormone uptake by target cells. Though it is generally assumed that steroid hormones enter their target cells by passive diffusion [17], it has been reported that steroid hormone uptake by intact tissues [IS] or by tumour cell lines [19,20], involves a temperature-dependent mechanism of transport through the...