The opiate family of receptors is believed to consist of the mu, delta, kappa and sigma subtypes. Autoradiographic studies have been conducted to define the distribution of the mu and delta receptors. However, due to the lack of a selective ligand definitive studies have not been reported for the kappa receptor. Recently such a ligand was discovered and characterized. Results are presented in which the distribution of kappa receptors in guinea pig brain slices were determined using autoradiographic techniques and its distribution compared to that of a typical mu ligand. The results demonstrate numerous differences between the two opioid systems. The kappa receptors predominate in the prefrontal cortex, the nucleus accumbens, and deep cortex; whereas the mu receptors have a more intense, broader and different distribution.