1. The functional expression of two potassium currents in cultured cerebellar granule cells was investigated with the whole cell patch-clamp technique in relation to development and growth condition. Cells were grown in medium containing different concentrations of potassium: 25 mM (K25) and 40 mM (K40), together referred to as "high K+"; 10 mM (K10) or "low K+"; and K10 with 100 microM N-methyl-D-aspartate (KNMDA). All conditions are known to influence maturation and survival of granule cells in culture. 2. At 2 days in vitro (DIV) the membrane capacitance, taken as index of membrane surface area, was the same for cells grown in each growth condition. At 7-9 DIV it had increased in each condition, but to a substantially larger extent in cells grown in KNMDA, K25, and K40 than in cells grown in K10. During development the input resistance only decreased in cells grown in KNMDA and high K+. 3. A delayed potassium current (IK) and a fast transient potassium current (IA) could both be recorded at 2 DIV in each growth condition, although a few neurons only expressed the IK. The IK was partially suppressed by tetraethylammonium (5 mM), whereas IA was predominantly sensitive to 4-aminopyridine (5 mM). 4. Normalized for cell capacitance, the specific IA conductance hardly changed during development in cells grown in high K+ and KNMDA. Cells in K10, however, displayed an IA with totally different properties in 23 of 24 cells; the specific IA conductance in these cells was considerably smaller at 7-9 DIV, suggesting a deletion of these channels during development.(ABSTRACT TRUNCATED AT 250 WORDS)