When I choose to weight nonlinear regression by 1/SD, why won't Prism give me any results?

Two possibilities: 1. It is impossible to weight by 1/SD if any of the SD values are 0 or blank. Or, if you entered replicates rather than SD values, it is impossible if any points have only a single replicate. 2. Prism won't weight by 1/SD if any of the Y values equal zero. You can work around this limitation by entering some small value (say, 0.000000001) instead of zero.