[python] matplotlib set yaxis label size

How can I change the size of only the yaxis label? Right now, I change the size of all labels using

pylab.rc('font', family='serif', size=40)

but in my case, I would like to make the y-axis label larger than the x-axis. However, I'd like to leave the tick labels alone.

I've tried, for example:

pylab.gca().get_ylabel().set_fontsize(60)

but I only get:

AttributeError: 'str' object has no attribute 'set_fontsize'

So, obviously that doesn't work. I've seen lots of stuff for tick sizes, but nothing for the axis labels themselves.