Our previous studies suggested that reduced choroidal blood flow (CBF) occurs with manipulations that yield myopic eye growth and that these reductions are primarily a consequence of the ocular enlargement. We could not entirely rule out the possibility, however, that reductions in CBF are at least to some extent antecedent and causal to the ocular enlargement. We therefore in the present study examined the effects on eye size of artificially reducing CBF by unilaterally transecting the choroidal nerves of the ciliary ganglion in four-day-old chicks. For comparison, we also transected the ciliary nerves in a second group of chicks or transected both ciliary and choroidal nerves in a third group of chicks. The effects of the nerve transections were evaluated in comparison to the effects of the orbital surgery itself (without nerve transection) in a fourth group termed the sham-operated control group. Two weeks after transection, CBF was measured using laser Doppler velocimetry, the ocular axial, nasotemporal and dorsoventral lengths were measured, and the eyes weighed. The results showed that CBF in birds with either choroidal nerve cuts or choroidal plus ciliary nerve cuts was greatly reduced in the treated eye (20-40% of nontreated eye). The treated eyes of these birds also showed gross depigmentation and histologically evident loss of the outer retina, most typically in the temporal retina. Birds with ciliary nerve cuts showed increased CBF in both eyes (131% right eye and 154% left eye compared to shams). Since ciliary nerve cuts yield fixed dilated pupils, increased CBF with ciliary nerve cuts appears consistent with the previously reported involvement of the choroidal nerves within a neural circuit subserving light-mediated upregulation of CBF. Clear effects on eye size were observed in the treated eyes in each group. The sham surgery alone yielded slight enlargement of the right eye compared to left eye, particularly in the axial dimension. In the choroidal nerve and the both nerve cut groups, nasotemporal and dorsoventral elongation were slightly diminished in the treated eyes compared to the sham-treated eyes. In contrast, enlargement of the right eye was slightly enhanced in the ciliary nerve cut group compared to the sham-treated eyes. The overall results suggest that large decreases in CBF do not enhance myopic eye growth, although large increases in CBF may.