Abstract 1. 1. Levels of progesterone and oestrone were estimated by means of RIA in the gonads and pyloric caeca of male specimens of Asterias rubens throughout the reproductive cycle. 2. 2. The patterns obtained appeared to be sex-specific on comparison with those of female specimens of A. rubens and it was concluded that progesterone and oestrone in the testes are involved in the regulation of gametogenesis. 3. 3. The levels of progesterone in the pyloric caeca were about ten times higher than those in the testes; in the same sequence levels of oestrone were only slightly higher. 4. 4. The onset of a new reproductive cycle, taking the abrupt decrease of the pyloric caeca-index as a marker, coincides with a strong decrease of the progesterone level ( P < 0.001) and an increase of the oestrone level ( P < 0.01) in the pyloric caeca. 5. 5. It is supposed that progesterone and oestrone in the pyloric caeca are concerned with making available the materials required for gametogenesis.