In previous work of our laboratory it was demonstrated that collidine (2,4,6-trimethylpyridine) abolishes the core osmiophilia and chromaffin reaction from rat pinal gland and vas deferens nerves. This abolition was apparent when tissues were briefly incubated in collidine or when they wer fixed in glutaraldehyde or osmium tetroxide using collidine as a buffer substance. These and other results strongly suggested that the histochemical effect of collidine was due to depletion of monoamines stored in the vesicles core. To examine this hypothesis we studied in this work the effect of collidine on tissues that have taken up tritiated noradrenaline. It was found that tritium was released very rapidly to the incubation medium when collidine was applied to fresh tissues. This effect was not observed with other commonly used buffers such as cacodylate or phosphate. It was also found that tritium release also occurred, although to a lesser extent, when tissues were fixed in glutaraldehyde or osmium tetroxide using collidine as a buffer, and this release was not significant when collidine was applied to previously fixed tissues. Paper chromatographic analysis showed that the radioactive compound(s) extracted from tissues by collidine corresponded to noradrenaline and/or closely related compounds. An abstract of this work was sent to the 17th Annual Meeting of the Society for Neuroscience, New Orleans, Nov 16-21, 1987. Tomsig J.L. and Pellegrino de Iraldi A. Abstract 369-11.