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SUMMARY1. Bleaching/regeneration cycles were performed in perfused frog retina while the optical transmittance at suitable wave-lengths was measured continuously. Rhodopsin was identified from its spectral absorbance, its photosensitivity and from the kinetics of its regeneration.2. In the absence of the pigment epithelium regeneration was complete when not more than 2-5 % of the rhodopsin initially present had been bleached. However, the cycles could be repeated to a total of regenerated rhodopsin exceeding that explicable on the utilization of stored chromophores. The rate of regeneration was fast, with 0-12 min-1 rate constant, following first order reaction kinetics. Under these conditions the cycle does not seem to involve stages beyond metarhodopsin II. With the moderate bleaching intensities used, half-time 53 min, the Bunsen-Roscoe law was obeyed up to 15 min, indicating a capacity for the photoproducts to be accommodated in situ for subsequent regeneration.3. It is concluded that only substantial bleaches, which exceed that capacity, result in hydrolysed chromophores. These surplus chromophores become esterified and are temporarily taken up by the pigment epithelium to be re-entered into the visual cycle as fast as they can be processed by the regenerative machinery of the rod outer segments.

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The effects of synthetic Angiotensin II on erythropoiesis were investigated. 1. Two daily intravenous injections of Angiotensin II, 100 µg./Kg. of body weight, revealed no acceleration of Fe59 incorporation into erythrocytes of either normal rabbits or hypophysectomized rats. 2. When given by intravenous drip, 100 µg./Kg. of Angiotensin II significantly accelerated the radioiron incorporation. 3. The renal blood flow was markedly reduced throughout the period of the dropwise injection; the same effect was transitory after single intravenous injection. The elevation of plasma erythropoietin activity was observed in hypertransfused polycythemic rabbits following the dropwise injection of Angiotensin II. From these results, it is concluded that erythropoietic activity of Angiotensin II results from an increased erythropoietin production. The increased production may have been induced by a renal ischemia through administration of Angiotensin II.