Summary. Using a method in which no substrate is added, the capacity of HIOMT to synthesize melatonin and/or 5-methoxytryptophol has been determined in the pineal and the eyes of the mole, a mammal having an atrophied visual system. In all animals studied, the activity of HIOMT in both eyes taken together was 2-10 times higher than in the pineal. The results prove that the pineal is not the only, and not always the most important, source of methoxyindoles.Introduction.

In pineals of 10 day old rats 5-hydroxytryptophan, 5-hydroxyindole-3-acetic acid, N-acetylserotonin/5-hydroxytryptophol and norepinephrine are methylated following a circadian rhythm. During the night HIOMT and COMT activities were measured for the above mentioned substrates, while HIOMT activity for 5-hydroxytryptophan and N-acetylserotonin/5-hydroxytryptophol was also determined during daytime.

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In order to obtain more information on the methylating capacity of the pineal gland, a method determining the formation of different 5-methoxyindoles in the pineal gland was developed. The method depends on measuring the incorporation of labelled methyl groups into the various hydroxyindoles present in the pineal gland, after incorporation of pineal tissue with labelled S-adenosyl methionine. Hydroxyindoles were not added to the incubation medium. After incubation thin-layer chromatography was performed with pineal tissue together with the incubation medium; the spots were scraped and counted.

The possible relation between haemopoietic activity and glycosaminoglycan levels in the haemopoietic microenvironment was examined in ectopic erythropoiesis, induced by phenylhydrazine treatment, in the liver of adult mice. The hepatic glycosaminoglycan content in the liver was determined biochemically over a period of 9 d following induction of haemolytic anaemia. After induction sulphated glycosaminoglycan levels increased up to d 4, then decreased to subnormal levels on d 5 and returned to normal values on the following days. The pattern of changes in GAG content coincided with changes in the number of CFU‐S and with the number of erythroblasts in the liver. This observation fits in with the hypothesis of McCuskey et al (1972) stating that high sulphated glycosaminoglycan levels are favourable for stem cell proliferation, whereas low sulphated glycosaminoglycan levels favour erythropoiesis.