1. 1. The effect of fetal calf serum on RNA and DNA synthesis in the 6-day chick embryo lens epithelium was investigated by electrophoresis, sucrose gradient analysis and CsCl buoyant density centrifugation. The cells in the excised lens epithelium elongate in culture when the medium is supplemented with fetal calf serum. 2. 2. Fetal calf serum stimulated the incorporation of [2,8- 3H 2]adenosine into polydisperse and 4-S RNA's by about 2-fold and into 28-S and 18-S ribosomal RNA's (rRNA) by approx. 4-fold in 3 h. These values were normalized on the basis of the specific activity of the intracellular pool of [ 3H]ATP. Experiments with actinomycin D (dactinomycin) showed that the greater incorporation was not due to slower degradation of RNA. 3. 3. After 3 h in the presence of l-[ Me- 3H]methionine, fetal calf serum increased the labeling of rRNA by about 3.8-fold. Fetal calf serum stimulated the rate of methylation of 45-S precursor rRNA and the accumulation of methylated 32-S, 28-S and 18-S RNA's while affecting neither the extent nor the rate of labeling of the S-adenosylmethionine pool. 4. 4. Fetal calf serum did not stimulate the rate of DNA synthesis, but possibly decreased it. 5. 5. These results show that the initiation of lens fiber formation in vitro by fetal calf serum is associated with an increase in the synthesis of all types of RNA, especially of rRNA.