The authors have examined the effect of mono- and di-methyl mercury on the shell thickness of two species, the Ring Dove (Streptopelia risoria) and American Kestrel (Falco sparverius). Two clutches (two eggs each) were collected from each pair of doves before the administration of mercury. The length and breadth of each egg was measured before the shell was ashed overnight at 1000/sup 0/. Ashed weight was considered to be a more accurate assay of calcium content than thickness for a species laying small eggs. The results for the Ring Doves are given and it is clear than no effect was observed. The kestrels' diet consisted of one day old dead chicks. The dead chicks were injured with dimethyl mercury dissolved in semane oil in the breast region immediately before being fed to the hawk. The kestrels were on diet for about three months before egg-laying commenced. The thickness of the shells remained exactly constant at 0.172 mm and the Ratcliffe Index (1) was 1.00 +/- 0.03 (10) for controls and 0.95 +/- 0.04 (x12) for those fed mercury. 11 references, 1 table.