The concentration of K+ [( K+]) was measured in the cerebrospinal fluid (CSF) and in the extracellular fluid [( K+]e) in the medial forebrain of two-day-old chicks by means of K(+)-sensitive microelectrodes (K-ISM). The K-ISM potential in the CSF was compared with that in artificial CSF with 3, 4 or 6 mmol/l of [K+]. The [K+] found in CSF was 3.79 +/- 0.57 (n = 8), in the hyperstriatum accessorium (HA) 3.70 +/- 0.32 (n = 13) and in the neostriatum (N) 3.51 +/- 0.32 (n = 13; mmol/l; mean +/- SEM). Trains of local electrical stimuli (10-100 Hz, 30 sec) applied to the surface of the forebrain increased [K+]e in both HA and N by 11-13 mmol/l. Increases in [K+]e in the ectostriatum (E) of 5-6 mmol/l was found in response to electrical stimulation (30-100 Hz, 5-10 sec) of the contralateral optic nerve, and of about 2 mmol/l by applying pressure to the bulb. In chicks adapted to the dark, stimulation of the contralateral eye or both eyes with bright light flashes (1-2 Hz) or with continuous light resulted in an increase in [K+]e of 0.5-1.0 mmol/l. Smaller increases in [K+]e of 0.15-1.25 mmol/l were found in HV and N after the application of a small quantity (0.1 ml) of the chemical taste aversant methylanthranilate (MeA) or the electrical stimulation of the beak by two needle electrodes inserted into the palatum or into the tongue. After application of MeA the increase in [K+]e began 3-4 min after application and persisted for 20 min.(ABSTRACT TRUNCATED AT 250 WORDS)