Somatostatin peptides are modified by substitution of para-methoxylated Phe or para-halogenated Phe for any Phe in the peptide. The modified somatostatin peptide analogs have the formulae: ##STR1## wherein R is hydrogen or an acyl group; R.sub.1 is selected from the group consisting of Lys or Des R.sub.1 ; R.sub.2 is selected from the group consisting of Asn, Ala or Des R.sub.2. The amino acids of the R.sub.2 group can be either the L-form or the D-form; R.sub.3 is selected from the group consisting of Trp or D-Trp; R.sub.4 is selected from the group consisting of Thr or Des R.sub.4 ; R.sub.5 is selected from the group consisting of Ser, D-Ser, Phe, X or Des R.sub.5 ; and X is selected from the group consisting of Phe, para-halogenated Phe and para-methoxylated Phe. Provided that at least one X is either para-halogenated Phe or para-methoxylated Phe.