A circadian variation in human cerebrospinal fluid (CSF) production has recently been demonstrated using magnetic resonance phase imaging. A nightly peak in CSF production was found at approximately 0200, when production is approximately twice the daytime values. In the present study, we have investigated the effect of a beta 1-receptor antagonist, atenolol, on the production of CSF, specifically the nocturnal production peak. CSF production was measured in fourteen healthy volunteers of both sexes in the time interval 1500-1800, with or without drug administration (100 mg orally) at 1800, and a second measurement was made in the time interval 2300-0200. In the absence of drug administration, all nine volunteers showed a significant increase in CSF production at night, from 0.34 +/- 0.06 ml/min in the time interval 1500-1800 to 0.61 +/- 0.05 (SE) ml/min (P < 0.005), confirming the presence of a circadian variation in these individuals. One week later, the experiment was repeated in five of these volunteers, plus an additional five volunteers, but with the administration of 100 mg atenolol orally immediately after the first measurement (at 1800). In five of the volunteers a decrease in CSF production was seen at midnight compared with daytime production values; in two volunteers CSF production remained unchanged, while three volunteers showed increased production. The average CSF production was 32% lower at night (0.27 +/- 0.10 ml/min) compared with the afternoon (0.40 +/- 0.07 ml/min), after administration of atenolol (P = 0.37).(ABSTRACT TRUNCATED AT 250 WORDS)