Here we report the use of fluorescence recovery after photobleaching (FRAP) to examine the intranuclear dynamics of fluorescent oestrogen receptor-α (ER). After bleaching, unliganded ER exhibits high mobility (recovery t 1/2< 1 s). Agonist (oestradiol; E2) or partial antagonist (4-hydroxytamoxifen) slows ER recovery (t 1/2∼ 5–6 s), whereas the pure antagonist (ICI 182,780) and, surprisingly, proteasome inhibitors each immobilize ER to the nuclear matrix. Dual FRAP experiments show that fluorescent ER and SRC-1 exhibit similar dynamics only in the presence of E2. In contrast to reports that several nuclear proteins show uniform dynamics, ER exhibits differential mobility depending upon several factors that are linked to its transcription function.