The administration of TCP (15 mg/kg, i.p.) to rats pretreated with either intraperitoneal RbCl (3 mmol/kg, twice daily for 5 days) or dietary RbCl (30 mmol/kg diet, for 14 days), resulted in the complete 5-HT behavioural syndrome. Pretreatment with p-chlorophenylalanine (i.p. 300 mg/kg x2) or (-)-propranolol (20 mg/kg, i.p.), pindolol (4 mg/kg, i.p.) and ritanserin (0.4 mg/kg, s.c.) prevented the occurrence of the 5-HT syndrome, produced by dietary RbCl plus TCP. Intraperitoneal administration of RbCl had no effect upon the 5-HT behavioural syndrome, produced by 8-OH-DPAT (0.5 mg/kg, s.c.) or 5-MeODMT (2 mg/kg, i.p.) but enhanced the 5-HT syndrome produced by quipazine (20 mg/kg, i.p.), DOI (8 mg/kg, s.c.), p-chloramphetamine (4 mg/kg, i.p.) or by TCP plus L-tryptophan (50 mg/kg, i.p.) in rats. Dietary administration of RbCl resulted in the enhancement of the 5-HT2-mediated head-twitches in the mouse and the attenuation of hypothermia in the mouse, induced by 8-OH-DPAT (0.5 mg/kg, s.c.). The accumulation of 5-HT (after inhibition of monoamine oxidase) and the rate of synthesis of 5-HT in the whole brain (minus cerebellum) were enhanced by dietary and intraperitoneal administration of RbCl, respectively. The effects of lithium and rubidium, respectively, on 5HT function in brain are compared.