Butylated hydroxytoluene (BHT) partitions effectively into lipids and modifies cellular function and behavior in numerous ways, particularly at low temperatures. In this study, the modification by BHT of the lifetime and dynamic rotational characteristics of diphenylhexatriene (DPH) in dipalmitoylphosphatidylcholine (DPPC) liposomes was investigated by phase fluorometry. BHT broadens the gel to liquid crystalline transition of DPPC by decreasing the lower end of the transition (Tt), leaving the upper end (Th) unaffected. The lifetime of DPH in DPPC does not vary linearly with temperature" but declines sharply above the main transition. The average lifetime of DPH is decreased to a greater extent below, compared to above, Th by BHT. This may be due to increased water penetration into the bilayer interior. Non-monoexponential decay also occurs below Th, probably due to a heterogeneous distribution of BHT, which would result in highly perturbed regions of greater than average BHT content. The motional parameter of DPF! most affected by BHT is the order parameter (S) which decreased to a considerable extent below Th but is not affected above Th. In contrast, the rotational diffusion coefficient (R) of DPH is increased slightly above Th by BFIT.