Cultured confluent rat astrocytes and C6 rat astrocytoma cells transport lysine with similar kinetic parameters. More than one transport mechanism may exist as suggested by discontinuities in the kinetic parameters. Sodium ion has a dual effect, being inhibitory between 1 and 10 mM and stimulatory between 30 and 137 mM. Exchange transport appears to be a minor component of lysine influx. The rate of transport into confluent cells is also affected by the recency of culture medium replacement, probably reflecting hormonal influences.