Theophylline concentrations in saliva and serum were compared in 20 patients with essential tremor. There was a strong correlation between the concentrations in saliva and serum (r = 0.9, slope of 0.48). The ratio between unbound and bound theophylline as assessed by ultrafiltration was 0.30. This ratio was similar to that between theophylline concentrations in CSF and serum. The theophylline concentration in serum was not predictable from that obtained in saliva in individual cases. Moreover, the levels in saliva were about 20% higher than the unbound concentrations of theophylline in serum.