Primary cultures from fetal rat brain contain angiotensin II immunoreactivity in neurons. To study the origin of this immunoreactivity, primary cultures were incubated with [3H]-valine or [3H]-isoleucine. A time-dependent incorporation of radioactivity into immunoprecipitable angiotensin II was observed. Incubation of cultures with captopril, an inhibitor of the converting enzyme, resulted in a significant inhibition of [3H]-valine incorporation into immunoreative [3H]-angiotensin II. Apparently, primary cultures from fetal rat brain can synthesize an angiotensin II-like immunoreactivity and support a concept that angiotensin II may be synthesized in fetal rat brain.