The results of an experiment in which 348 Raja clavata were tagged and injected with tetracycline in November 1968 are described. This drug is deposited in calcified tissues and fluoresces in ultraviolet light, and can therefore be used as a time marker in calcified skeletal structures. There were 86 recaptures from the experiment by mid-October 1971, but only 16 fish were actually returned. Examinations of these showed that tetracycline was deposited in all the calcified skeletal structures present at the time of injection. It was not found in the secondary sexual alar and malar spines of a male ray which was immature at tagging and mature at recapture. The deposition of tetracycline in relation to the opaque and hyaline zones occurring on the centra of the vertebrae is described in detail. Six of these rays had just started the formation of a hyaline zone at the time of injection, and six were still forming an opaque zone; one ray was in a transitional stage. These observations, combined with those on the appearance of the opaque zone at the periphery of the centrum, show that formation of the opaque zone probably occurs in the latter half of the year but usually does not become visible, when the centra are observed by reflected light, until the early months of the subsequent year. By relating the opaque and hyaline zones to the deposition of tetracycline it is proved that one of each type of zone is laid down annually. The difficulties in interpreting the edge structure in relation to the use of the centra for age determination are discussed.