The effects of adenosine and adenine nucleotides on a calcium-activated non-selective cation channel, present in the plasma membrane of an insulin-secreting cell line CRI-Gl were investigated. Single-channel currents were recorded from inside-out membrane patches and the adenine derivatives applied to the solution bathing the cytoplasmic aspect of the membrane surface. The activity of this channel is shown to be inhibited by all the derivatives tested. The potency sequence for inhibition was found to be AMP greater than ADP greater than ATP greater than adenosine.