I would like to compare two groups, one treated, one control.

Usually, I would simply use a t-test, assuming normal distribution

and equal variance.

However, I would now like to use another parameter as a co-factor:

I am comparing two groups of animals and the effect I observe

could be due to a difference in weight.

Which test can account for this? I know that if I had three groups

or more I could use one-way ANOVA, but since I only have two groups...

Probably, this is a very basic question, I'm just not very firm with stats.

Thanks for your help