Hep.
Any ideas how can I measure micro amperes with Arduino?
Range will be from 0,5 micro ampere to about 5 milliamperes, but mostly something between 1 and 5 uA.
Voltage from 0.7 to 2 volts, variable and easily controlled by user via potentiometer or something like that.
Probably linear regulator to control the voltage, because target is to measure quartz movement of a wristwatc: smallest amount of voltage which will run it, and current consumption to see the condition of movement.
-eemeli-
.. and don't ask for: "which opamp"? 
Low power, low input current bias
sigh
maybe something like the LMV301
Hep.
Thank You very much!
never done anything with opamps, so one question about those: which value range will work best for R2? I suppose it to be less than 10 ohms, so please correct if I am completely out of range?
I may use 7 segment led display to tell uA:s to me, but that I have to google next week - seems to be lots of information I can study myself.
-eemeli-
Have you spotted the formula?
U = R * (-I)
So you might want U = 5Volts for your ADC for a current of 100uA.
-> R = 5/0.0001 =50 k (or 47 k)
Note that the resolution of the ADC is limited! It will not be possible to measure around 1uA and 1mA with the same resistor!!
Using 47k will give you a voltage at the ADC for 0.5 uA of about 5 readings only (resolution 0.1uA).
Another thing: If it is not feasable to let the current flow out of the opamp into your device, you have to install a virtual ground, by e.g. connecting two 10k resistors as voltage devider from 5V and connect the 2.5V to opamps + input rather than the true ground