Maybe you also need \runMPTEXgraphicstrue (enabled in my cont-sys.tex)?
I just enabled that in cont-sys.rme, and I put it in the test file for good measure.
your example works here (beta 17.09.). I also tried your example at contextgarden (same beta) and it worked too (using Firefox).
Thanks for checking it both ways. Now I'm really puzzled because I also use Firefox (on Linux). I just ran the file below at contextgarden (same one as before but with the MPTEX addition), and it gives 'undefined' in a square where the figure should be. And on my Ubuntu laptop with context 2006.08.08 it gives a large black circle. I also tried the old perl texexec in case that helped but no luck. The texexec log on contextgarden looks okay. It generates texexec-mpgraph.1 and embeds one figure, so I'm guessing the figure is generated and inserted. So I don't understand why the generated PDF file is not right. To make sure it wasn't a caching problem, I added a bit of text to the end, to see whether the new PDF file had it (it did). \runMPgraphicstrue \runMPTEXgraphicstrue \setupcolors[state=start] \starttext \startreusableMPgraphic{a} fill fullcircle scaled 10cm withcolor transparent(1,0.5,green) ; \stopreusableMPgraphic \placefigure[force,none]{}{\reuseMPgraphic{a}} \stoptext -Sanjoy