A probe for a drug receptor site can be assembled from a fragment (D) derived from a drug of proven biological activity, a connection of variable length and character (Cn), and a moiety (Y)capable of reactivity toward a limited set of functional groups (F) at or near the receptor site. Covalent bond formation between Y and F should result in labeling of the receptor site, making possible its isolation. If the biological effects of the D-Cn-YF combination are similar to those of D, long-term activity could result. A number of derivatives of barbital and d-amphetamine with Cn = C2, C3, C3, or C2OC2 and Y = 2,3-dimethylmaleimidyl are described, together with the results of a limited series of biological tests (rats, firefly larvae). Many of the compds exhibit activity, indicating that the Cn substituent on D does not abolish the interaction of D with the receptor sites (which may not be the same as those involved with the action of the drug D). The duration of action of the derivatives is not prolonged, implying that an SH group is not readily accessible at or near the receptor sites.