excuse please my keyboard is about to be replaced and I cannot type (successor(y)) character. I shall substitute with v. In routine pks if v is non-0 and small the argument to exp(-1.2337etc/SQR(v)) will be large and the exp will underflow to 0. This will make the sqrt(-log(y)) non-finite. Apologies if this has already been discussed.

Yes, you are right. Thanks for finding this. The fix is now posted here (http://nr.com/forum/showthread.php?t=2038).