The neurotensin receptors from rat brain synaptosomal membranes differed in subunit structure from those in rat gastric fundus smooth muscle plasma membranes when studied following photoaffinity labelling or after exposure to cross-linking reagents. However, these two receptors were similar in their recognition properties. In this study we compared the target size of the two receptors by radiation inactivation and observed that the receptors in both tissues had similar target sizes (mean values 103,000 and 108,000 daltons). This suggests that the differences in size observed in biochemical studies may reflect changes occurring during the isolation procedures, or, on the other hand, there might be inherent difference in the subunit structure of these receptors.