The molecular organization and dynamics have been investigated in membranes consisting of 1-palmitoyl-2-oleoyl-l-alpha-phosphatidylcholine (POPC) and various ratios of a transmembrane alpha-helical peptide, Ac-K2L24K2-amide (L24), in order to gain insights into how the transmembrane portions of membrane proteins are mixed with phospholipids and organized in biological membranes. Particular attention was paid to membranes with high peptide concentrations. The molecular organization and dynamics were studied in the ps-to-micros regime using various spin-labeling techniques. Conventional ESR spectra as well as saturation-recovery curves measured in both the presence and the absence of molecular oxygen showed that phosphatidylcholine spin-labels detect the existence of a single homogeneous environment, indicating that both L24 and POPC are likely to be undergoing fast translational diffusion in L24-POPC membranes of up to 9 mol % peptide. Since 16-18 molecules of phosphatidylcholine are required to surround a transmembrane alpha-helical peptide [Morrow, M. R., Huschilt, J. C., and Davis, J. H. (1985) Biochemistry 24, 5396-5406], L24 must form L24-rich regions at a P/L ratio of 1/10 instantaneously. However, these results suggest that the lipid exchange rates among the bulk, boundary, and L24-rich regions are fast, and that the L24-rich regions must be forming and dispersing rapidly in a time scale shorter than 0.1 micros, the conventional ESR spin-label time scale and the electron spin-lattice relaxation time scale in the presence of molecular oxygen. Although this does not exclude the possibility of the formation of small, stable oligomers of L24, it is unlikely because L24 lacks features that would favor their formation. L24 (9 mol %) increases the hydrophobicity of the central part of the POPC membrane from the level of 1-decanol to that of pure hexane and also increases the hydrophobicity near the membrane surface from the level of 2-propanol to that of 1-decanol. The effect of 9 mol % L24 on the order parameter profile is similar to that of decreasing the temperature by approximately 8 degrees C between 10 and 55 degrees C. It is concluded that L24 is highly miscible in POPC membranes even at high concentrations in the membrane.