The effect of intracameral injection of 1.0 micrograms of substance P (SP) on the regional ocular blood flow in albino rabbits was investigated by a method using radioactively labelled microspheres. The mean ciliary blood flow in SP-treated eyes was 0.163 +/- 0.006 g/min and in the control eyes 0.107 +/- 0.004 g/min. The flow increase was 72 +/- 22%. The mean difference was 0.056 +/- 0.005 g/min (P less than 0.01, n = 11). Infusion of 25-40 micrograms of substance P into one common carotid artery over 20-45 min caused a rise in intraocular pressure of 22.5 +/- 1.5 cm H2O in the ipsilateral eye and of 1.6 +/- 0.2 cm H2O in the contralateral one. The mean difference was 19 +/- 5.3 cm H2O (P less than 0.05, n = 4). The protein concentration of the aqueous humour on the ipsilateral side was higher than on the contralateral one, and there was marked extravasation of intravenously injected Evans blue in the ciliary processes in the ipsilateral eyes. Extravasation of Evans blue int he ciliary processes and a rise in intraocular pressure also occurred in two rabbits which were given in intravenous injection of SP (0.37 or 3.4 mg). It is concluded that in rabbits SP tends to increase the intraocular pressure and to cause a breakdown of the blood-aqueous barrier and that the increase in ciliary blood flow caused by SP may play a role in these mechanisms.