1 Using a radioimmunoassay, plasma fluphenazine (FPZ) concentrations were examined in 33 schizophrenic patients during 38 intervals between injections of FPZ decanoate. Doses ranged from 12.5 to 150 mg and intervals from 1 to 5 weeks. At least three blood samples were taken between injections from each subject; also in 26 subjects additional samples were taken during the first 24 h post-injection. 2 FPZ was measurable in all plasma samples. 3 Each injection was followed by a rapid rise in plasma FPZ concentration to a maximum at 1-8 h. The height of this peak varied considerably. Within the next 12-36 h plasma FPZ fell to a level slightly above that found before injection and then remained stable until the next injection, thus confirming the steady release of FPZ from the depot over this period. 4 For the group, dose and mean plasma FPZ levels correlated strongly. 5 Despite this, there was a four-fold variation in plasma FPZ concentration among subjects receiving the same dose. 6 The FPZ level on the last day of an interval between injections was a satisfactory estimate of the mean FPZ level for the interval. 7 In one subject examined in this way, a positive correlation was found (r = 0.76) between plasma FPZ (by radioimmunoassay) and plasma prolactin levels.