Red blood cell (RBC) and plasma concentrations of haloperidol (H) and reduced haloperidol (RH) were determined in nine patients (five men, four women) who had been receiving monthly injections of haloperidol decanoate (HD) for at least 3 months. A very sensitive high performance liquid chromatography method was used to measure H and RH, which could be detected at levels as low as 0J ng/ml. Blood samples for H and RH were drawn at the end of weeks 1, 2, 3, and 4 of a 4-week cyde. Monthly HD dosages ranged from 50 mg to 200 mg. At each time point RBC H did not differ significantly from plasma H. RBC RH and plasma RH did not change significantly over time and were consistently lower than RBC H and plasma H. However, RBC RH levels were higher than plasma RH levels. Also, the ratios of RBC RH/H were higher than were the ratios of plasma RH/H, which reflect an accumulation of RH intracellularly. HD dosage was highly correlated with both RBC H and plasma H, but RBC RH and plasma RH were not significantly related to dosage at any time point. Significant correlations were observed between HD dosage and prolactin concentration at week 1 and between prolactin concentration and RBC H and plasma H at weeks 1 and 4. Blood concentrations of H and RH and the corresponcling standardized areas under the curve were not significantly related to smoking.