The spontaneous formation of ordered spatial concentration patterns in an unstirred chemical medium, supported by dissipation of chemical free energy, has been considered often since a pioneering suggestion by Turing and early work by Prigogine and more recent work by Ross involving nonequilibrium thermodynamics. The prototype experimental example is the oscillatory Belousov-Zhabotinsky reaction, in which target patterns of outward-moving concentric rings are readily observed. One widely-studied question is whether "microscopic" fluctuations can nucleate these target centers, or whether a catalytic, nucleating heterogeneous center is required. Vidal and Pagola observed spontaneous initiation with no nucleating particles visible at 6-micron resolution; however Zhang, Forster, and Ross argued theoretically that this is impossible in regimes far from Hopf bifurcations. We describe here an explicit mechanism in a "supercritical regime," following and near to the low-f Hopf bifurcation in a generalized Oregonator model, by which microscopic fluctuations can nucleate activity, and reconcile these results with Zhang Concentrations remain very close to the unstable steady-state values after the system slowly passes through the bifurcation point but before occurrence of the inevitable transition to large-amplitude limit cycle oscillations. Suitably timed small (even microscopic) fluctuations about this supercritical state can sharply accelerate the inevitable onset of large-amplitude limit cycle oscillations, potentially nucleating targets. (C) 2003 American Institute of Physics.