Haloperidol concentrations in saliva and serum were measured by radioimmunoassay using conjugate III-bovine serum albumin as an antigen. The results were as follows: (1) There was a statistically significant correlation between saliva and serum levels, with the former being higher than the latter. (2) There were significant correlations between daily dosage and salivary and serum concentrations, and the regression lines were shown in the equivalent exponential form. (3) Serum and salivary concentrations remained constant over a 24-hour period in a steady state. (4) The saliva-serum ratio was inversely proportional to salivary pH, but there was no significant correlation between the ratio and the volume of saliva.