Morning variations of plasma homovanillic acid in untreated schizophrenic patients.

Plasma concentrations of homovanillic acid were determined in samples obtained at 8.30 a.m. and 12.30 p.m. from 29 untreated schizophrenic patients, 14 males and 15 females. When the earlier samples were compared with the later, a significant decrease in mean plasma homovanillic acid level was observed, but only in the male patients. The morning fall was observed in 10 of 14 male patients and 6 of the 15 female patients. This morning rhythm in plasma homovanillic acid concentration may mask the putative rises in plasma homovanillic acid provoked by neuroleptic administration and may explain some of the observed differences between findings in studies involving the assessment of this metabolite.