Substance P (SP)-ergic neurons from 16/17 day-embryonic rat brain stem in primary culture were identified by immunocytochemistry using biotinylated avidin and phosphatase alkaline methods with affinity-purified anti-SP antibodies. An average of 84% of neurons contained SP from day 9 to day 21 after plating. These in vitro data show that SP-containing neurons develop in our culture conditions. SP may act as a maturation factor as well as a neurotransmitter.