Trigeminal and dorsal root ganglia were excised from mouse embryos of 10 and 18 days in utero age, respectively, and grown in tissue culture. A quantitative method was used to assess the extent of neurite outgrowth from the explants after 24 and 48 h in culture. Outgrowth from ganglia grown in a serumless medium was compared with that from ganglia in a medium supplemented with serum. There was no significant difference between the extent of outgrowth in either medium after 24 h in culture; however, after 48 h there was significantly greater outgrowth in the serum-supplemented medium.