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1. A study has been made of the cause of the temporal disparity between the pupillary and intraocular pressure responses to noradrenaline applied topically. 2. Over a 5-day experimental period, the quantitative characteristics of the pupillary response to noradrenaline applied either once or several times daily remained essentially unchanged. In the same rabbits the delay in pressure response to noradrenaline on the first day was increased by a second application and, on subsequent days, multiple applications of noradrenaline induced a biphasic pressure response. 3. Intravenous phenoxybenzamine inhibited both the pupillary and the biphasic pressure responses to noradrenaline. Phenoxybenzamine applied topically inhibited the ocular hypertensive but not the hypotensive and pupillary responses. 4. Analysis of aqueous humour dynamics by manometric procedures showed that the hyper- and hypotensive responses were associated with increase and decrease, respectively, of the resistance to outflow of the aqueous humour.

1. A theoretical and experimental analysis of the relationship of the corneal stromal ultrastructure with light transmission has been made in an attempt to resolve recent contradictory explanations of corneal transparency.2. The spatial distribution of collagen fibrils in electronmicrographs of rabbit corneal stroma has been analysed in terms of a radial distribution function. The results indicate the presence of local order extending to at least 200 nm from individual fibrils.3. The observed spatial distribution of the collagen fibrils was used as a basis to compare the theoretically derived and the experimentally determined values of light transmission. It has been found that the transparency of the normal cornea may be explained by the quasi-random structure revealed by the electronmicroscope.4. Histograms of the collagen fibril diameter in normal rabbit corneal stroma revealed the range to be 12.5-32.5 nm and the mean value to be approximately 20 +/- 1.5 nm. Corneal swelling did not change the collagen fibril diameter significantly.5. It is concluded that the size and distribution of collagen fibrils revealed in electronmicrographs are consistent with the observed transparency of normal stromas.6. A marked heterogeneity in the spatial distribution of collagen fibrils was found in the swollen cornea. This is qualitatively consistent with the observed decrease in transparency.