Abstract 1. 1. The isolated crayfish vas deferens is highly specific and sensitive for l-glutamate (threshold approximately 10 −6 g. per ml), giving a phasic contraction. 2. 2. Rapid desensitization may explain the phasic nature of the contraction. 3. 3. The organ contracts tonically to γ-aminobutyrate (GABA). 4. 4. Other potential transmitter substances are without effect, so the vas deferens may contain only GABA and l-glutamate receptors.