Technical Abstract: Grapefruit (Citrus x paradisi) develop symptoms of chilling injury (CI) if held at temperatures below about 10 degrees C. Conditioning grapefruit at a low, but non-chilling temperature prior to storage at a chilling temperature reduces the development of CI symptoms. Changes in transcript abundance for a number of genes have been correlated with chilling stress in citrus fruit. We tested the hypothesis that conditioning affects transcript abundance of genes related to chilling stress in grapefruit. Grapefruit were harvested from a commercial grove in Florida in September and divided into two groups; one group was placed immediately at 5 degrees C (non-conditioned (NC)); the second group was placed at 16 degrees C for one week (conditioned (C)) and then transferred to 5 degrees C. Symptoms of CI were visible on NC and C fruit following 14 days at 5 degrees C, but were consistently more severe on NC than C fruit. Storage at 5 degrees C caused increases in abundance of transcripts for 1-aminocyclopropane-1-carboxylate oxidase (ACO), galactinol synthase, a chilling induced oxygenase, and a temperature induced-lipocalin, consistent with previous reports. Levels of these transcripts were lower in C than in NC fruit during storage at 5 degrees C, but this pattern did not persist following transfer to 20 degrees C. Levels of transcripts for catalase, a metallothionein-like protein, a lipid transfer protein, a stress-responsive zinc finger protein, and a citrus low temperature-inducible protein were consistently higher in C than NC fruit during storage at 5 degrees C. Our results confirm that conditioning increases chilling tolerance in grapefruit and demonstrates that abundance of transcripts of a number of genes related to chilling stress is affected by conditioning. We cannot say, however, that these changes are related to chilling tolerance per se; they may only reflect the difference between chilling-sensitive and chilling-tolerant fruit.